SQL-Server Questions
What is RDBMS?
Relational
Data Base Management Systems (RDBMS) are database management systems that
maintain data records and indices in tables. Relationships may be created and maintained
across and among the data and tables. In a relational database, relationships
between data items are expressed by means of tables. Interdependencies among
these tables are expressed by data values rather than by pointers. This allows
a high degree of data independence. An RDBMS has the capability to recombine
the data items from different files, providing powerful tools for data usage.
What is normalization?
Database
normalization is a data design and organization processes applied to data
structures based on rules that help build relational databases. In relational
database design, the process of organizing data to minimize redundancy.
Normalization usually involves dividing a database into two or more tables and
defining relationships between the tables. The objective is to isolate data so
that additions, deletions, and modifications of a field can be made in just one
table and then propagated through the rest of the database via the defined
relationships.
What are different normalization forms?
1NF: Eliminate Repeating GroupsMake a separate table for each set of related
attributes, and give each table a primary key. Each field contains at most one
value from its attribute domain.
2NF:
Eliminate Redundant DataIf an attribute depends on only part of a multi-valued
key, remove it to a separate table.
3NF:
Eliminate Columns Not Dependent On Key If attributes do not contribute to a
description of the key, remove them to a separate table. All attributes must be
directly dependent on the primary key
BCNF:
Boyce-Codd Normal FormIf there are non-trivial dependencies between candidate
key attributes, separate them out into distinct tables.
4NF:
Isolate Independent Multiple RelationshipsNo table may contain two or more 1:n
or n:m relationships that are not directly related.
5NF:
Isolate Semantically Related Multiple Relationships There may be practical
constrains on information that justify separating logically related
many-to-many relationships.
ONF:
Optimal Normal FormA model limited to only simple (elemental) facts, as
expressed in Object Role Model notation.
DKNF:
Domain-Key Normal FormA model free from all modification anomalies.
Remember, these normalization guidelines are cumulative. For a database to be
in 3NF, it must first fulfill all the criteria of a 2NF and 1NF database.
What is Stored Procedure?
A
stored procedure is a named group of SQL statements that have been previously
created and stored in the server database. Stored procedures accept input
parameters so that a single procedure can be used over the network by several
clients using different input data. And when the procedure is modified, all
clients automatically get the new version. Stored procedures reduce network
traffic and improve performance. Stored procedures can be used to help ensure
the integrity of the database.e.g. sp_helpdb, sp_renamedb, sp_depends etc.
What is Trigger?
A
trigger is a SQL procedure that initiates an action when an event (INSERT,
DELETE or UPDATE) occurs. Triggers are stored in and managed by the
DBMS.Triggers are used to maintain the referential integrity of data by
changing the data in a systematic fashion. A trigger cannot be called or
executed; the DBMS automatically fires the trigger as a result of a data
modification to the associated table. Triggers can be viewed as similar to
stored procedures in that both consist of procedural logic that is stored at
the database level. Stored procedures, however, are not event-drive and are not
attached to a specific table as triggers are. Stored procedures are explicitly
executed by invoking a CALL to the procedure while triggers are implicitly
executed. In addition, triggers can also execute stored procedures.Nested
Trigger: A trigger can also contain INSERT, UPDATE and DELETE logic within
itself, so when the trigger is fired because of data modification it can also
cause another data modification, thereby firing another trigger. A trigger that
contains data modification logic within itself is called a nested trigger.
What is View?
A
simple view can be thought of as a subset of a table. It can be used for
retrieving data, as well as updating or deleting rows. Rows updated or deleted
in the view are updated or deleted in the table the view was created with. It
should also be noted that as data in the original table changes, so does data in
the view, as views are the way to look at part of the original table. The
results of using a view are not permanently stored in the database. The data
accessed through a view is actually constructed using standard T-SQL select
command and can come from one to many different base tables or even other
views.
What is Index?
An
index is a physical structure containing pointers to the data. Indices are
created in an existing table to locate rows more quickly and efficiently. It is
possible to create an index on one or more columns of a table, and each index
is given a name. The users cannot see the indexes, they are just used to speed
up queries. Effective indexes are one of the best ways to improve performance
in a database application. A table scan happens when there is no index
available to help a query. In a table scan SQL Server examines every row in the
table to satisfy the query results. Table scans are sometimes unavoidable, but
on large tables, scans have a terrific impact on performance.
Clustered indexes define the physical sorting of a database table’s rows in the
storage media. For this reason, each database table may have only one clustered
index.Non-clustered indexes are created outside of the database table and
contain a sorted list of references to the table itself.
What is the difference between clustered and a non-clustered index?
A
clustered index is a special type of index that reorders the way records in the
table are physically stored. Therefore table can have only one clustered index.
The leaf nodes of a clustered index contain the data pages.
A nonclustered index is a special type of index in which the logical order of
the index does not match the physical stored order of the rows on disk. The
leaf node of a nonclustered index does not consist of the data pages. Instead,
the leaf nodes contain index rows.
What are the different index configurations a table can have?
A table
can have one of the following index configurations:
No indexesA clustered indexA clustered index and many nonclustered indexesA
nonclustered indexMany nonclustered indexes
What is cursors?
Cursor
is a database object used by applications to manipulate data in a set on a
row-by-row basis, instead of the typical SQL commands that operate on all the
rows in the set at one time.
In order to work with a cursor we need to perform some steps in the following
order:
Declare cursorOpen cursorFetch row from the cursorProcess fetched rowClose
cursorDeallocate cursor
What is the use of DBCC commands?
DBCC
stands for database consistency checker. We use these commands to check the
consistency of the databases, i.e., maintenance, validation task and status
checks.E.g. DBCC CHECKDB - Ensures that tables in the db and the indexes are
correctly linked.DBCC CHECKALLOC - To check that all pages in a db are
correctly allocated.DBCC CHECKFILEGROUP - Checks all tables file group for any
damage.
What is a Linked Server?
Linked
Servers is a concept in SQL Server by which we can add other SQL Server to a
Group and query both the SQL Server dbs using T-SQL Statements. With a linked
server, you can create very clean, easy to follow, SQL statements that allow
remote data to be retrieved, joined and combined with local data.Storped
Procedure sp_addlinkedserver, sp_addlinkedsrvlogin will be used add new Linked
Server.
What is Collation?
Collation
refers to a set of rules that determine how data is sorted and compared.
Character data is sorted using rules that define the correct character
sequence, with options for specifying case-sensitivity, accent marks, kana character
types and character width.
What are different type of Collation Sensitivity?
Case
sensitivityA and a, B and b, etc.
Accent sensitivitya and á, o and ó, etc.
Kana SensitivityWhen Japanese kana characters Hiragana and Katakana are treated
differently, it is called Kana sensitive.
Width sensitivityWhen a single-byte character (half-width) and the same
character when represented as a double-byte character (full-width) are treated
differently then it is width sensitive.
What’s the difference between a primary key and a unique key?
Both
primary key and unique enforce uniqueness of the column on which they are
defined. But by default primary key creates a clustered index on the column,
where are unique creates a nonclustered index by default. Another major difference
is that, primary key doesn’t allow NULLs, but unique key allows one NULL only.
How to
implement one-to-one, one-to-many and many-to-many relationships while
designing tables?
One-to-One
relationship can be implemented as a single table and rarely as two tables with
primary and foreign key relationships.One-to-Many relationships are implemented
by splitting the data into two tables with primary key and foreign key
relationships.Many-to-Many relationships are implemented using a junction table
with the keys from both the tables forming the composite primary key of the
junction table.
What is a NOLOCK?
Using
the NOLOCK query optimiser hint is generally considered good practice in order
to improve concurrency on a busy system. When the NOLOCK hint is included in a
SELECT statement, no locks are taken when data is read. The result is a Dirty
Read, which means that another process could be updating the data at the exact
time you are reading it. There are no guarantees that your query will retrieve
the most recent data. The advantage to performance is that your reading of data
will not block updates from taking place, and updates will not block your
reading of data. SELECT statements take Shared (Read) locks. This means that
multiple SELECT statements are allowed simultaneous access, but other processes
are blocked from modifying the data. The updates will queue until all the reads
have completed, and reads requested after the update will wait for the updates
to complete. The result to your system is delay(blocking).
What is difference between DELETE & TRUNCATE commands?
Delete
command removes the rows from a table based on the condition that we provide
with a WHERE clause. Truncate will actually remove all the rows from a table
and there will be no data in the table after we run the truncate command.
TRUNCATETRUNCATE is faster and uses fewer system and transaction log resources
than DELETE.TRUNCATE removes the data by deallocating the data pages used to
store the table’s data, and only the page deallocations are recorded in the
transaction log.TRUNCATE removes all rows from a table, but the table structure
and its columns, constraints, indexes and so on remain. The counter used by an
identity for new rows is reset to the seed for the column.You cannot use TRUNCATE
TABLE on a table referenced by a FOREIGN KEY constraint.Because TRUNCATE TABLE
is not logged, it cannot activate a trigger.TRUNCATE can not be Rolled back
using logs.TRUNCATE is DDL Command.TRUNCATE Resets identity of the table.
DELETEDELETE removes rows one at a time and records an entry in the transaction
log for each deleted row.If you want to retain the identity counter, use DELETE
instead. If you want to remove table definition and its data, use the DROP
TABLE statement.DELETE Can be used with or without a WHERE clauseDELETE
Activates Triggers.DELETE Can be Rolled back using logs.DELETE is DML
Command.DELETE does not reset identity of the table.
Difference between Function and Stored Procedure?
UDF can
be used in the SQL statements anywhere in the WHERE/HAVING/SELECT section where
as Stored procedures cannot be.UDFs that return tables can be treated as
another rowset. This can be used in JOINs with other tables.Inline UDF’s can be
though of as views that take parameters and can be used in JOINs and other
Rowset operations.
When is the use of UPDATE_STATISTICS command?
This
command is basically used when a large processing of data has occurred. If a
large amount of deletions any modification or Bulk Copy into the tables has
occurred, it has to update the indexes to take these changes into account.
UPDATE_STATISTICS updates the indexes on these tables accordingly.
What types of Joins are possible with Sql Server?
Joins
are used in queries to explain how different tables are related. Joins also let
you select data from a table depending upon data from another table.Types of
joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further
classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS.
What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?
Specifies
a search condition for a group or an aggregate. HAVING can be used only with
the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP
BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically
used only with the GROUP BY function in a query. WHERE Clause is applied to
each row before they are part of the GROUP BY function in a query. HAVING
criteria is applied after the the grouping of rows has occurred.
What is sub-query?
Explain
properties of sub-query.Sub-queries are often referred to as sub-selects, as
they allow a SELECT statement to be executed arbitrarily within the body of
another SQL statement. A sub-query is executed by enclosing it in a set of
parentheses. Sub-queries are generally used to return a single row as an atomic
value, though they may be used to compare values against multiple rows with the
IN keyword.
A subquery is a SELECT statement that is nested within another T-SQL statement.
A subquery SELECT statement if executed independently of the T-SQL statement,
in which it is nested, will return a result set. Meaning a subquery SELECT
statement can standalone and is not depended on the statement in which it is
nested. A subquery SELECT statement can return any number of values, and can be
found in, the column list of a SELECT statement, a FROM, GROUP BY, HAVING,
and/or ORDER BY clauses of a T-SQL statement. A Subquery can also be used as a
parameter to a function call. Basically a subquery can be used anywhere an
expression can be used.
Properties of Sub-QueryA subquery must be enclosed in the parenthesis.A
subquery must be put in the right hand of the comparison operator, andA
subquery cannot contain a ORDER-BY clause.A query can contain more than one
sub-queries.
What are types of sub-queries?
Single-row
subquery, where the subquery returns only one row.Multiple-row subquery, where
the subquery returns multiple rows,.andMultiple column subquery, where the
subquery returns multiple columns.
What is SQL Profiler?
SQL
Profiler is a graphical tool that allows system administrators to monitor
events in an instance of Microsoft SQL Server. You can capture and save data
about each event to a file or SQL Server table to analyze later. For example,
you can monitor a production environment to see which stored procedures are
hampering performances by executing too slowly.
Use SQL Profiler to monitor only the events in which you are interested. If
traces are becoming too large, you can filter them based on the information you
want, so that only a subset of the event data is collected. Monitoring too many
events adds overhead to the server and the monitoring process and can cause the
trace file or trace table to grow very large, especially when the monitoring
process takes place over a long period of time.
What is User Defined Functions?
User-Defined
Functions allow to define its own T-SQL functions that can accept 0 or more
parameters and return a single scalar data value or a table data type.
What kind of User-Defined Functions can
be created?
There
are three types of User-Defined functions in SQL Server 2000 and they are
Scalar, Inline Table-Valued and Multi-statement Table-valued.
Scalar User-Defined FunctionA Scalar user-defined function returns one of the
scalar data types. Text, ntext, image and timestamp data types are not
supported. These are the type of user-defined functions that most developers
are used to in other programming languages. You pass in 0 to many parameters
and you get a return value.
Inline Table-Value User-Defined FunctionAn Inline Table-Value user-defined
function returns a table data type and is an exceptional alternative to a view
as the user-defined function can pass parameters into a T-SQL select command
and in essence provide us with a parameterized, non-updateable view of the
underlying tables.
Multi-statement Table-Value User-Defined FunctionA Multi-Statement Table-Value
user-defined function returns a table and is also an exceptional alternative to
a view as the function can support multiple T-SQL statements to build the final
result where the view is limited to a single SELECT statement. Also, the
ability to pass parameters into a T-SQL select command or a group of them gives
us the capability to in essence create a parameterized, non-updateable view of
the data in the underlying tables. Within the create function command you must
define the table structure that is being returned. After creating this type of
user-defined function, It can be used in the FROM clause of a T-SQL command
unlike the behavior found when using a stored procedure which can also return
record sets.
Which TCP/IP port does SQL Server run on? How can it be changed?
SQL
Server runs on port 1433. It can be changed from the Network Utility TCP/IP
properties –> Port number.both on client and the server.
What are the authentication modes in SQL Server? How can it be changed?Windows
mode and mixed mode (SQL & Windows).
To change authentication mode in SQL Server click Start, Programs, and
Microsoft SQL Server and click SQL Enterprise Manager to run SQL Enterprise
Manager from the Microsoft SQL Server program group. Select the server then
from the Tools menu select SQL Server Configuration Properties, and choose the
Security page.
Where SQL server user’s names and passwords are are stored in sql
server?
They
get stored in master db in the sysxlogins table.
Which command using Query Analyzer will give you the version of SQL
server and operating system?
SELECT
SERVERPROPERTY ('productversion'), SERVERPROPERTY ('productlevel'),
SERVERPROPERTY ('edition')
What is SQL server agent?
SQL
Server agent plays an important role in the day-to-day tasks of a database
administrator (DBA). It is often overlooked as one of the main tools for SQL
Server management. Its purpose is to ease the implementation of tasks for the
DBA, with its full-function scheduling engine, which allows you to schedule
your own jobs and scripts.
Can a stored procedure call itself or recursive stored procedure? How
many levels SP nesting possible?
Yes.
Because Transact-SQL supports recursion, you can write stored procedures that
call themselves. Recursion can be defined as a method of problem solving
wherein the solution is arrived at by repetitively applying it to subsets of
the problem. A common application of recursive logic is to perform numeric computations
that lend themselves to repetitive evaluation by the same processing steps.
Stored procedures are nested when one stored procedure calls another or
executes managed code by referencing a CLR routine, type, or aggregate. You can
nest stored procedures and managed code references up to 32 levels.
What is @@ERROR?
The
@@ERROR automatic variable returns the error code of the last Transact-SQL
statement. If there was no error, @@ERROR returns zero. Because @@ERROR is
reset after each Transact-SQL statement, it must be saved to a variable if it
is needed to process it further after checking it.
What is Raiseerror?
Stored
procedures report errors to client applications via the RAISERROR command.
RAISERROR doesn’t change the flow of a procedure; it merely displays an error
message, sets the @@ERROR automatic variable, and optionally writes the message
to the SQL Server error log and the NT application event log.
What is log shipping?
Log
shipping is the process of automating the backup of database and transaction
log files on a production SQL server, and then restoring them onto a standby
server. Enterprise Editions only supports log shipping. In log shipping the
transactional log file from one server is automatically updated into the backup
database on the other server. If one server fails, the other server will have
the same db can be used this as the Disaster Recovery plan. The key feature of
log shipping is those is will automatically backup transaction logs throughout
the day and automatically restore them on the standby server at defined
interval.
What is the difference between a local and a global variable?
A local
temporary table exists only for the duration of a connection or, if defined
inside a compound statement, for the duration of the compound statement.
A global temporary table remains in the database permanently, but the rows
exist only within a given connection. When connections are closed, the data in
the global temporary table disappears. However, the table definition remains
with the database for access when database is opened next time.
What command do we use to rename a db?
sp_renamedb
‘oldname’, ‘newname’If someone is using db it will not accept sp_renmaedb. In
that case first bring db to single user using sp_dboptions. Use sp_renamedb to
rename database. Use sp_dboptions to bring database to multi user mode.
What are sp_configure commands and set commands?
Use
sp_configure to display or change server-level settings. To change
database-level settings, use ALTER DATABASE. To change settings that affect
only the current user session, use the SET statement.
What are the different types of replication?
Explain.The
SQL Server 2000-supported replication types are as follows:
Transactional
Snapshot
Merge
Snapshot
replication distributes data exactly as it appears at a specific moment in time
and does not monitor for updates to the data. Snapshot replication is best used
as a method for replicating data that changes infrequently or where the most
up-to-date values (low latency) are not a requirement. When synchronization
occurs, the entire snapshot is generated and sent to Subscribers.
Transactional replication, an initial snapshot of data is applied at
Subscribers, and then when data modifications are made at the Publisher, the
individual transactions are captured and propagated to Subscribers.
Merge replication is the process of distributing data from Publisher to
Subscribers, allowing the Publisher and Subscribers to make updates while
connected or disconnected, and then merging the updates between sites when they
are connected.
What are the OS services that the SQL Server installation adds?
MS SQL SERVER SERVICE, SQL AGENT SERVICE, DTC (Distribution transac
co-ordinator)
What are three SQL keywords used to change or set someone’s permissions?
GRANT,
DENY, and REVOKE.
What does it mean to have quoted_identifier on? What are the
implications of having it off?
When
SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double quotation
marks, and literals must be delimited by single quotation marks. When SET
QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all
Transact-SQL rules for identifiers.
What is the STUFF function and how does it differ from the REPLACE
function?
STUFF
function to overwrite existing characters. Using this syntax, STUFF
(string_expression, start, length, replacement_characters), string_expression
is the string that will have characters substituted, start is the starting
position, length is the number of characters in the string that are
substituted, and replacement_characters are the new characters interjected into
the string.REPLACE function to replace existing characters of all occurance.
Using this syntax REPLACE (string_expression, search_string,
replacement_string), where every incidence of search_string found in the
string_expression will be replaced with replacement_string.
Using query analyzer, name 3 ways to get an accurate count of the number
of records in a table?
SELECT * FROM table1SELECT COUNT (*) FROM table1SELECT
rows FROM sysindexes WHERE id = OBJECT_ID (table1) AND indid < 2
How to rebuild Master Database?
Shutdown
Microsoft SQL Server 2000 and then run Rebuildm.exe. This is located in the
Program Files\Microsoft SQL Server\80\Tools\Binn directory.In the Rebuild
Master dialog box, click Browse.In the Browse for Folder dialog box, select the
\Data folder on the SQL Server 2000 compact disc or in the shared network
directory from which SQL Server 2000 was installed, and then click OK.Click
Settings. In the Collation Settings dialog box, verify or change settings used
for the master database and all other databases.Initially, the default
collation settings are shown, but these may not match the collation selected
during setup. You can select the same settings used during setup or select new
collation settings. When done, click OK.In the Rebuild Master dialog box, click
Rebuild to start the process.The Rebuild Master utility reinstalls the master
database.To continue, you may need to stop a server that is running.Source:
What are the basic functions for master, msdb, model, tempdb databases?
The
Master database holds information for all databases located on the SQL Server
instance and is the glue that holds the engine together. Because SQL Server
cannot start without a functioning master database, you must administer this
database with care.The msdb database stores information regarding database
backups, SQL Agent information, DTS packages, SQL Server jobs, and some
replication information such as for log shipping.The tempdb holds temporary
objects such as global and local temporary tables and stored procedures.The
model is essentially a template database used in the creation of any new user
database created in the instance.
What are primary keys and foreign keys?
Primary keys are the unique identifiers for
each row. They must contain unique values and cannot be null. Due to their
importance in relational databases, Primary keys are the most fundamental of
all keys and constraints. A table can have only one Primary key.Foreign keys
are both a method of ensuring data integrity and a manifestation of the
relationship between tables.
What is data integrity?
Explain
constraints?Data integrity is an important feature in SQL Server. When used
properly, it ensures that data is accurate, correct, and valid. It also acts as
a trap for otherwise undetectable bugs within applications.
A
PRIMARY KEY constraint is a unique identifier for a row within a database
table. Every table should have a primary key constraint to uniquely identify
each row and only one primary key constraint can be created for each table. The
primary key constraints are used to enforce entity integrity.
A
UNIQUE constraint enforces the uniqueness of the values in a set of columns, so
no duplicate values are entered. The unique key constraints are used to enforce
entity integrity as the primary key constraints.
A
FOREIGN KEY constraint prevents any actions that would destroy links between
tables with the corresponding data values. A foreign key in one table points to
a primary key in another table. Foreign keys prevent actions that would leave
rows with foreign key values when there are no primary keys with that value.
The foreign key constraints are used to enforce referential integrity.
A CHECK
constraint is used to limit the values that can be placed in a column. The
check constraints are used to enforce domain integrity.
A NOT
NULL constraint enforces that the column will not accept null values. The not
null constraints are used to enforce domain integrity, as the check
constraints.
What are the properties of the Relational tables?
Relational
tables have six properties:
Values are atomic.
Column values are of the same kind.
Each row is unique.
The sequence of columns is insignificant.
The sequence of rows is insignificant.
Each column must have a unique name.
What is De-normalization?
De-normalization
is the process of attempting to optimize the performance of a database by
adding redundant data. It is sometimes necessary because current DBMSs implement
the relational model poorly. A true relational DBMS would allow for a fully
normalized database at the logical level, while providing physical storage of
data that is tuned for high performance. De-normalization is a technique to
move from higher to lower normal forms of database modeling in order to speed
up database access.
How to get @@error and @@rowcount at the same time?
If
@@Rowcount is checked after Error checking statement then it will have 0 as the
value of @@Recordcount as it would have been reset.And if @@Recordcount is
checked before the error-checking statement then @@Error would get reset. To
get @@error and @@rowcount at the same time do both in same statement and store
them in local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR
What is Identity?
Identity
(or AutoNumber) is a column that automatically generates numeric values. A
start and increment value can be set, but most DBA leave these at 1. A GUID
column also generates numbers; the value of this cannot be controled.
Identity/GUID columns do not need to be indexed.
What is a Scheduled Jobs or What is a Scheduled Tasks?
Scheduled tasks let user automate processes that run on regular or
predictable cycles. User can schedule administrative tasks, such as cube
processing, to run during times of slow business activity. User can also
determine the order in which tasks run by creating job steps within a SQL
Server Agent job. E.g. back up database, Update Stats of Tables. Job steps give
user control over flow of execution. If one
job fails, user can configure SQL Server Agent to continue to run the remaining
tasks or to stop execution.
What is a table called, if it does have neither Cluster nor Non-cluster
Index? What is it used for?
Unindexed
table or Heap. Microsoft Press Books and Book on Line (BOL) refers it as Heap.A
heap is a table that does not have a clustered index and, therefore, the pages
are not linked by pointers. The IAM pages are the only structures that link the
pages in a table together.Unindexed tables are good for fast storing of data.
Many times it is better to drop all indexes from table and than do bulk of
inserts and to restore those indexes after that.
What is BCP? When does it used?
BulkCopy
is a tool used to copy huge amount of data from tables and views. BCP does not
copy the structures same as source to destination.
How do you load large data to the SQL server database?
BulkCopy
is a tool used to copy huge amount of data from tables. BULK INSERT command
helps to Imports a data file into a database table or view in a user-specified
format.
Can we rewrite subqueries into simple select statements or with joins?
Subqueries
can often be re-written to use a standard outer join, resulting in faster
performance. As we may know, an outer join uses the plus sign (+) operator to tell
the database to return all non-matching rows with NULL values. Hence we combine
the outer join with a NULL test in the WHERE clause to reproduce the result set
without using a sub-query.
Can SQL Servers linked to other servers like Oracle?
SQL
Server can be lined to any server provided it has OLE-DB provider from
Microsoft to allow a link. E.g. Oracle has an OLE-DB provider for oracle that
Microsoft provides to add it as linked server to SQL Server group.
How to know which index a table is using?
SELECT
table_name, index_name FROM user_constraints
How to
copy the tables, schema and views from one SQL server to another?
Microsoft
SQL Server 2000 Data Transformation Services (DTS) is a set of graphical tools
and programmable objects that lets user extract, transform, and consolidate
data from disparate sources into single or multiple destinations.
What is Self Join?
This is
a particular case when one table joins to itself, with one or two aliases to
avoid confusion. A self join can be of any type, as long as the joined tables
are the same. A self join is rather unique in that it involves a relationship
with only one table. The common example is when company have a hierarchal
reporting structure whereby one member of staff reports to another.
What is Cross Join?
A cross
join that does not have a WHERE clause produces the Cartesian product of the
tables involved in the join. The size of a Cartesian product result set is the
number of rows in the first table multiplied by the number of rows in the
second table. The common example is when company wants to combine each product
with a pricing table to analyze each product at each price.
Which virtual table does a trigger use?
Inserted
and Deleted.
List
few advantages of Stored Procedure.
Ø Stored procedure can reduced network traffic and latency, boosting
application performance.
Ø Stored procedure execution plans can be reused, staying cached in SQL
Server’s memory, reducing server overhead.
Ø Stored procedures help promote code reuse.
Ø Stored procedures can encapsulate logic. You can change stored procedure
code without affecting clients.
Ø Stored procedures provide better security to your data.
What is DataWarehousing?
Subject-oriented, meaning that the data in the database is organized so that
all the data elements relating to the same real-world event or object are
linked together;
Time-variant, meaning that the changes to the data in the database are tracked
and recorded so that reports can be produced showing changes over time;
Non-volatile, meaning that data in the database is never over-written or
deleted, once committed, the data is static, read-only, but retained for future
reporting;
Integrated, meaning that the database contains data from most or all of an
organization’s operational applications, and that this data is made consistent.
What is OLTP (OnLine Transaction Processing)?
In OLTP
- online transaction processing systems relational database design use the
discipline of data modeling and generally follow the Codd rules of data
normalization in order to ensure absolute data integrity. Using these rules
complex information is broken down into its most simple structures (a table)
where all of the individual atomic level elements relate to each other and
satisfy the normalization rules.
How do SQL server 2000 and XML linked?
Can XML
be used to access data?FOR XML (ROW, AUTO, EXPLICIT)You can execute SQL queries
against existing relational databases to return results as XML rather than
standard rowsets. These queries can be executed directly or from within stored
procedures. To retrieve XML results, use the FOR XML clause of the SELECT
statement and specify an XML mode of RAW, AUTO, or EXPLICIT.
OPENXMLOPENXML
is a Transact-SQL keyword that provides a relational/rowset view over an
in-memory XML document. OPENXML is a rowset provider similar to a table or a
view. OPENXML provides a way to access XML data within the Transact-SQL context
by transferring data from an XML document into the relational tables. Thus,
OPENXML allows you to manage an XML document and its interaction with the
relational environment.
What is an execution plan? When would
you use it?
How
would you view the execution plan?An execution plan is basically a road map
that graphically or textually shows the data retrieval methods chosen by the
SQL Server query optimizer for a stored procedure or ad-hoc query and is a very
useful tool for a developer to understand the performance characteristics of a
query or stored procedure since the plan is the one that SQL Server will place
in its cache and use to execute the stored procedure or query. From within
Query Analyzer is an option called “Show Execution Plan” (located on the Query
drop-down menu). If this option is turned on it will display query execution
plan in separate window when query is ran again.
How do you implement one-to-one, one-to-many and many-to-many
relationships while designing tables?
One-to-One
relationship can be implemented as a single table and rarely as two tables with
primary and foreign key relationships. One-to-Many relationships are implemented
by splitting the data into two tables with primary key and foreign key
relationships. Many-to-Many relationships are implemented using a junction
table with the keys from both the tables forming the composite primary key of
the junction table. It will be a good idea to read up a database designing
fundamentals text book.
What’s the difference between a primary key and a unique key?
Both
primary key and unique enforce uniqueness of the column on which they are
defined. But by default primary key creates a clustered index on the column,
where are unique creates a nonclustered index by default. Another major
difference is that, primary key doesn’t allow NULLs, but unique key allows one
NULL only.
What is user defined data types and when you should go for them?
User
defined data types let you extend the base SQL Server data types by providing a
descriptive name, and format to the database. Take for example, in your
database, there is a column called Flight_Num which appears in many tables. In
all these tables it should be varchar (8). In this case you could create a user
defined data type called Flight_num_type of varchar (8) and use it across all
your tables. See sp_addtype, sp_droptype in books online.
What is bit data type and what’s the information that can be stored
inside a bit column?
Bit
data type is used to store Boolean information like 1 or 0 (true or false).
Until SQL Server 6.5 bit data type could hold either a 1 or 0 and there was no
support for NULL. But from SQL Server 7.0 onwards, bit data type can represent
a third state, which is NULL.
Define candidate key, alternate key, and composite key.
A
candidate key is one that can identify each row of a table uniquely. Generally
a candidate key becomes the primary key of the table. If the table has more
than one candidate key, one of them will become the primary key, and the rest
are called alternate keys. A key formed by combining at least two or more
columns is called composite key.
What are defaults? Is there a column to which a default can’t be bound?
A
default is a value that will be used by a column, if no value is supplied to
that column while inserting data. IDENTITY columns and timestamp columns can’t
have defaults bound to them. See CREATE DEFAULT in books online.
What is a transaction and what are ACID properties?
A
transaction is a logical unit of work in which, all the steps must be performed
or none. ACID stands for Atomicity, Consistency, Isolation, and Durability.
These are the properties of a transaction. For more information and explanation
of these properties, see SQL Server books online or any RDBMS fundamentals text
book. Explain different isolation levels An isolation level determines the
degree of isolation of data between concurrent transactions. The default SQL
Server isolation level is Read Committed. Here are the other isolation levels
(in the ascending order of isolation): Read Uncommitted, Read Committed,
Repeatable Read, and Serializable. See SQL Server books online for an
explanation of the isolation levels. Be sure to read about SET TRANSACTION
ISOLATION LEVEL, which lets you customize the isolation level at the connection
level. Read Committed - A transaction operating at the Read Committed level
cannot see changes made by other transactions until those transactions are committed.
At this level of isolation, dirty reads are not possible but nonrepeatable
reads and phantoms are possible. Read Uncommitted - A transaction operating at
the Read Uncommitted level can see uncommitted changes made by other
transactions. At this level of isolation, dirty reads, nonrepeatable reads, and
phantoms are all possible. Repeatable Read - A transaction operating at the
Repeatable Read level is guaranteed not to see any changes made by other
transactions in values it has already read. At this level of isolation, dirty
reads and nonrepeatable reads are not possible but phantoms are possible.
Serializable - A transaction operating at the Serializable level guarantees
that all concurrent transactions interact only in ways that produce the same effect
as if each transaction were entirely executed one after the other. At this
isolation level, dirty reads, nonrepeatable reads, and phantoms are not
possible.
CREATE INDEX my Index ON my Table (my Column) what type of Index will
get created after executing the above statement?
Non-clustered
index. Important thing to note: By default a clustered index gets created on
the primary key, unless specified otherwise.
What’s the maximum size of a row?
8060 bytes. Don’t be surprised with questions like ‘what is the maximum number
of columns per table’. 1024 columns per table. Check out SQL Server books
online for the page titled: "Maximum Capacity Specifications".
Explain Active/Active and Active/Passive cluster configurations hopefully you
have experience setting up cluster servers. But if you don’t, at least be
familiar with the way clustering works and the two clustering configurations
Active/Active and Active/Passive. SQL Server books online has enough
information on this topic and there is a good white paper available on
Microsoft site.
Explain
the architecture of SQL Server This is a very important question and you better
be able to answer it if consider yourself a DBA. SQL Server books online is the
best place to read about SQL Server architecture. Read up the chapter dedicated
to SQL Server Architecture.
What is lock escalation?
Lock escalation is the process of converting a lot of low level locks (like row
locks, page locks) into higher level locks (like table locks). Every lock is a
memory structure too many locks would mean, more memory being occupied by
locks. To prevent this from happening, SQL Server escalates the many fine-grain
locks to fewer coarse-grain locks. Lock escalation threshold was definable in
SQL Server 6.5, but from SQL Server 7.0 onwards it’s dynamically managed by SQL
Server.
What’s the difference between DELETE
TABLE and TRUNCATE TABLE commands?
DELETE TABLE is a logged operation, so the deletion of each row gets logged in
the transaction log, which makes it slow. TRUNCATE TABLE also deletes all the
rows in a table, but it won’t log the deletion of each row, instead it logs the
deallocation of the data pages of the table, which makes it faster. Of course,
TRUNCATE TABLE can be rolled back. TRUNCATE TABLE is functionally identical to
DELETE statement with no WHERE clause: both remove all rows in the table. But
TRUNCATE TABLE is faster and uses fewer system and transaction log resources
than DELETE. The DELETE statement removes rows one at a time and records an
entry in the transaction log for each deleted row. TRUNCATE TABLE removes the
data by deallocating the data pages used to store the table’s data, and only
the page deallocations are recorded in the transaction log. TRUNCATE TABLE
removes all rows from a table, but the table structure and its columns,
constraints, indexes and so on remain. The counter used by an identity for new
rows is reset to the seed for the column. If you want to retain the identity
counter, use DELETE instead. If you want to remove table definition and its
data, use the DROP TABLE statement. You cannot use TRUNCATE TABLE on a table
referenced by a FOREIGN KEY constraint; instead, use DELETE statement without a
WHERE clause. Because TRUNCATE TABLE is not logged, it cannot activate a
trigger. TRUNCATE TABLE may not be used on tables participating in an indexed
view
Explain the storage models of OLAP
Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more
information.
What are the new features introduced in SQL Server 2000 (or the latest release
of SQL Server at the time of your interview)? What changed between the previous
version of SQL Server and the current version?
This question is generally asked to see how current your knowledge is.
Generally there is a section in the beginning of the books online titled
"What’s New", which has all such information. Of course, reading just
that is not enough; you should have tried those things to better answer the
questions. Also check out the section titled "Backward Compatibility"
in books online which talks about the changes that have taken place in the new
version.
What are constraints? Explain different
types of constraints.
Constraints enable the RDBMS enforce the integrity of the database
automatically, without needing you to create triggers, rule or defaults. Types
of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY. For an
explanation of these constraints see books online for the pages titled:
"Constraints" and "CREATE TABLE", "ALTER TABLE"
What is an index? What are the types of
indexes? How many clustered indexes can be created on a table?
I create a separate index on each column of a table. What are the
advantages and disadvantages of this approach?
Indexes in SQL Server are similar to the indexes in books. They help SQL Server
retrieve the data quicker. Indexes are of two types. Clustered indexes and
non-clustered indexes.
When
you create a clustered index on a table, all the rows in the table are stored
in the order of the clustered index key. So, there can be only one clustered
index per table. Non-clustered indexes have their own storage separate from the
table data storage. Non-clustered indexes are stored as B-tree structures (so
do clustered indexes), with the leaf level nodes having the index key and its
row locater. The row located could be the RID or the Clustered index key,
depending up on the absence or presence of clustered index on the table. If you
create an index on each column of a table, it improves the query performance,
as the query optimizer can choose from all the existing indexes to come up with
an efficient execution plan. At the same time, data modification operations
(such as INSERT, UPDATE, DELETE) will become slow, as every time data changes
in the table, all the indexes need to be updated. Another disadvantage is that,
indexes need disk space, the more indexes you have, more disk space is used.
What is RAID and what are different
types of RAID configurations?
RAID stands for Redundant Array of Inexpensive Disks, used to provide fault
tolerance to database servers. There are six RAID levels 0 through 5 offering
different levels of performance, fault tolerance. MSDN has some information
about RAID levels and for detailed information, check out the RAID advisory
board’s homepage
What are the steps you will take to improve performance of a poor
performing query?
This is
a very open ended question and there could be a lot of reasons behind the poor
performance of a query. But some general issues that you could talk about would
be: No indexes, table scans, missing or out of date statistics, blocking,
excess recompilations of stored procedures, procedures and triggers without SET
NOCOUNT ON, poorly written query with unnecessarily complicated joins, too much
normalization, excess usage of cursors and temporary tables. Some of the tools/ways
that help you troubleshooting performance problems are: SET SHOWPLAN_ALL ON,
SET SHOWPLAN_TEXT ON, SET STATISTICS IO ON, SQL Server Profiler, Windows NT
/2000 Performance monitor, Graphical execution plan in Query Analyzer. Download
the white paper on performance tuning SQL Server from Microsoft web site. Don’t
forget to check out sql-server-performance.com
What are the steps you will take, if you are tasked with securing an SQL
Server?
Again
this is another open ended question. Here are some things you could talk about:
Preferring NT authentication, using server, database and application roles to
control access to the data, securing the physical database files using NTFS
permissions, using an unguessable
SA
password, restricting physical access to the SQL Server, renaming the
Administrator account on the SQL Server computer, disabling the Guest account,
enabling auditing, using multiprotocol encryption, setting up SSL, setting up
firewalls, isolating SQL Server from the web server etc. Read the white paper
on SQL Server security from Microsoft website. Also check out My SQL Server
security best practices
What is a deadlock and what is a live lock? How will you go about
resolving deadlocks?
Deadlock
is a situation when two processes, each having a lock on one piece of data,
attempt to acquire a lock on the other’s piece. Each process would wait
indefinitely for the other to release the lock, unless one of the user
processes is terminated. SQL Server detects deadlocks and terminates one user’s
process. A livelock is one, where a request for an exclusive lock is repeatedly
denied because a series of overlapping shared locks keeps interfering. SQL
Server detects the situation after four denials and refuses further shared
locks. A livelock also occurs when read transactions monopolize a table or
page, forcing a write transaction to wait indefinitely. Check out SET
DEADLOCK_PRIORITY and "Minimizing Deadlocks" in SQL Server books
online. Also check out the article Q169960 from Microsoft knowledge base.
What is blocking and how would you troubleshoot it?
Blocking
happens when one connection from an application holds a lock and a second
connection requires a conflicting lock type. This forces the second connection
to wait, blocked on the first. Read up the following topics in SQL Server books
online: Understanding and avoiding blocking, Coding efficient transactions.
Explain CREATE
DATABASE
syntax Many of us are used to creating databases from the Enterprise Manager or
by just issuing the command:
CREATE
DATABAE MyDB.
But
what if you have to create a database with two file groups, one on drive C and
the other on drive D with log on drive E with an initial size of 600 MB and
with a growth factor of 15%?
That’s
why being a DBA you should be familiar with the CREATE DATABASE syntax. Check
out SQL Server books online for more information.
How to
restart SQL Server in single user mode? How to start SQL Server in minimal
configuration mode?
SQL
Server can be started from command line, using the SQLSERVR.EXE. This EXE has
some very important parameters with which a DBA should be familiar with. -m is
used for starting SQL Server in single user mode and -f is used to start the
SQL Server in minimal configuration mode. Check out SQL Server books online for
more parameters and their explanations.
As a part of your job, what are the DBCC commands that you commonly use
for database maintenance?
DBCC
CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC, DBCC SHOWCONTIG,
DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But there are a whole load of DBCC
commands which are very useful for DBAs.
Check
out SQL Server books online for more information.
What are statistics, under what circumstances they go out of date, and
how do you update them?
Statistics
determine the selectivity of the indexes. If an indexed column has unique
values then the selectivity of that index is more, as opposed to an index with
non-unique values. Query optimizer uses these indexes in determining whether to
choose an index or not while executing a query. Some situations under which you
should update statistics:
1) If
there is significant change in the key values in the index
2) If a
large amount of data in an indexed column has been added, changed, or removed
(that is, if the distribution of key values has changed), or the table has been
truncated using the TRUNCATE TABLE statement and then repopulated
3)
Database is upgraded from a previous version. Look up SQL Server books online
for the following commands: UPDATE STATISTICS, STATS_DATE, DBCC
SHOW_STATISTICS, CREATE STATISTICS, DROP STATISTICS, sp_autostats,
sp_createstats, and sp_updatestats
What are the different ways of moving data/databases between servers and
databases in SQL Server?
There
are lots of options available; you have to choose your option depending upon
your requirements. Some of the options you have are: BACKUP/RESTORE, detaching
and attaching databases, replication, DTS, BCP, log shipping, INSERT…SELECT,
SELECT…INTO, creating INSERT scripts to generate data.
Explain different types of BACKUPs available in SQL Server? Given a
particular scenario, how would you go about choosing a backup plan?
Types
of backups you can create in SQL Sever 7.0+ are Full database backup,
differential database backup, transaction log backup, file group backup. Check
out the BACKUP and RESTORE commands in SQL Server books online. Be prepared to
write the commands in your interview. Books online also has information on
detailed backup/restore architecture and when one should go for a particular
kind of backup.
What is database replication? What are the different types of
replication you can set up in SQL Server?
Replication
is the process of copying/moving data between databases on the same or
different servers.
SQL Server supports the following types of replication scenarios:?
Snapshot replication?
Transactional
replication (with immediate updating subscribers, with queued updating
subscribers)?
Merge
replication See SQL Server books online for in depth coverage on replication.
Be prepared to explain how different replication agents function, what are the
main system tables used in replication etc.
How to determine the service pack currently installed on SQL Server?
The
global variable @@Version stores the build number of the sqlservr.exe, which is
used to determine the service pack installed. To know more about this process
visit SQL Server service packs and versions.
What are cursors? Explain different types of cursors. What are the
disadvantages of cursors? How can you avoid cursors?
Cursors allow row-by-row processing of the result sets. Types of cursors:
Static, Dynamic, Forward-only, Keyset-driven. See books online for more
information. Disadvantages of cursors: Each time you fetch a row from the
cursor, it results in a network roundtrip; where as a normal SELECT query makes
only one roundtrip, however large the result set is. Cursors are also costly
because they require more resources and temporary storage (results in more IO
operations). Further, there are restrictions on the SELECT statements that can
be used with some types of cursors. Most of the times, set based operations can
be used instead of cursors. Here is an example: If you have to give a flat hike
to your employees using the following criteria: Salary between 30000 and 40000
— 5000 hike Salary between 40000 and 55000 — 7000 hike Salary between 55000 and
65000 — 9000 hike. In this situation many developers tend to use a cursor,
determine each employee’s salary and update his salary according to the above
formula. But the same can be achieved by multiple update statements or can be
combined in a single UPDATE statement as shown below:
UPDATE tbl_emp SET salary = CASE WHEN salary BETWEEN 30000 AND
40000 THEN salary +
5000 WHEN salary BETWEEN
40000 AND 55000 THEN
salary + 7000 WHEN
salary BETWEEN 55000 AND
65000 THEN salary +
10000 END
Another
situation in which developers tend to use cursors: You need to call a stored
procedure when a column in a particular row meets certain condition. You don’t
have to use cursors for this. This can be achieved using WHILE loop, as long as
there is a unique key to identify each row. For examples of using WHILE loop
for row by row processing, check out the ‘My code library’ section of my site
or search for WHILE. Write down the general syntax for a SELECT statement
covering all the options. Here’s the basic syntax: (Also checkout SELECT in
books online for advanced syntax).
SELECT select list [INTO new table_] FROM table source [WHERE search condition] [GROUP BY group_by_expression]
[HAVING search condition] [ORDER BY order expression [ASC DESC] ]
What is a join and explain different
types of joins.
Joins are used in queries to explain how different tables are related. Joins
also let you select data from a table depending upon data from another table.
Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further
classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS. For
more information see pages from books online titled: "Join
Fundamentals" and "Using Joins".
Can you have a nested transaction?
Yes, very
much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT
What is an extended stored procedure? Can you instantiate a COM object
by using T-SQL?
An
extended stored procedure is a function within a DLL (written in a programming
language like C, C++ using Open Data Services (ODS) API) that can be called
from T-SQL, just the way we call normal stored procedures using the EXEC
statement. See books online to learn how to create extended stored procedures
and how to add them to SQL Server. Yes, you can instantiate a COM (written in
languages like VB, VC++) object from T-SQL by using sp_OACreate stored
procedure. Also see books online for sp_OAMethod, sp_OAGetProperty,
sp_OASetProperty, sp_OADestroy. For an example of creating a COM object in VB and
calling it from T-SQL, see ‘My code library’ section of this site.
What is the system function to get the current user’s user id?
USER_ID
(). Also check out other system functions like USER_NAME (), SYSTEM_USER,
SESSION_USER, CURRENT_USER, USER, SUSER_SID (), HOST_NAME ().
What are triggers? How many triggers you can have on a table? How to
invoke a trigger on demand?
Triggers
are special kind of stored procedures that get executed automatically when an
INSERT, UPDATE or DELETE operation takes place on a table. In SQL Server 6.5
you could define only 3 triggers per table, one for INSERT, one for UPDATE and
one for DELETE. From SQL Server 7.0 onwards, this restriction is gone, and you
could create multiple triggers per each action. But in 7.0 there’s no way to
control the order in which the triggers fire. In SQL Server 2000 you could
specify which trigger fires first or fires last using sp_settriggerorder.
Triggers can’t be invoked on demand. They get triggered only when an associated
action (INSERT, UPDATE, and DELETE) happens on the table on which they are
defined. Triggers are generally used to implement business rules, auditing.
Triggers can also be used to extend the referential integrity checks, but
wherever possible, use constraints for this purpose, instead of triggers, as
constraints are much faster. Till SQL Server 7.0, triggers fire only after the
data modification operation happens. So in a way, they are called post
triggers. But in SQL Server 2000 you could create pre triggers also. Search SQL
Server 2000 books online for INSTEAD OF triggers. Also check out books online
for ‘inserted table’, ‘deleted table’ and COLUMNS_UPDATED ()
There
is a trigger defined for INSERT operations on a table, in an OLTP system. The
trigger is written to instantiate a COM object and pass the newly inserted rows
to it for some custom processing.
What do you think of this implementation? Can this be implemented
better?
Instantiating
COM objects is a time consuming process and since you are doing it from within
a trigger, it slows down the data insertion process. Same is the case with
sending emails from triggers. This scenario can be better implemented by
logging all the necessary data into a separate table, and have a job which
periodically checks this table and does the needful.
What is a self join? Explain it with an example.
Self
join is just like any other join, except that two instances of the same table
will be joined in the query. Here is an example: Employees table which contains
rows for normal employees as well as managers. So, to find out the managers of
all the employees, you need a self join.
CREATE TABLE emp ( empid int, mgrid int, empname char(10) )
INSERT emp SELECT 1,2,’Vyas’ INSERT emp SELECT 2,3,’Mohan’
INSERT emp SELECT 3,NULL,’Shobha’
INSERT emp SELECT 4,2,’Shridhar’
INSERT emp SELECT 5,2,’Sourabh’
SELECT t1.empname [Employee],
t2.empname [Manager] FROM
emp t1, emp t2 WHERE
t1.mgrid = t2.empid
Here’s an advanced query using a LEFT OUTER JOIN that even returns the employees without
managers (super
bosses)
SELECT t1.empname [Employee], COALESCE(t2.empname, ‘No manager’) [Manager] FROM emp t1 LEFT OUTER JOIN emp t2 ON t1.mgrid = t2.empid
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