SQL server Interview Questions
What is difference between DELETE and 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.
TRUNCATE:
TRUNCATE 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, its columns, constraints, indexes and so on, remains. 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 cannot be rolled back.
TRUNCATE is DDL Command.
TRUNCATE Resets identity of the table
DELETE:
DELETE 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 clause
DELETE Activates Triggers.
DELETE can be rolled back.
DELETE is DML Command.
DELETE does not reset identity of the table.
Note: DELETE and TRUNCATE both can be rolled back when surrounded by TRANSACTION if the current session is not closed. If TRUNCATE is written in Query Editor
surrounded by TRANSACTION and if session is closed, it can not be rolled back but DELETE can be rolled back.
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 is the difference between a HAVING CLAUSE and a WHERE CLAUSE?
They specify a search condition for a group or an aggregate. But the difference is that 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
whereas WHERE Clause is applied to each row before they are part of the GROUP BY function in a query.
What are the properties and different Types of Sub-Queries?
Properties of Sub-Query
A sub-query must be enclosed in the parenthesis.
A sub-query must be put in the right hand of the comparison operator, and
A sub-query cannot contain an ORDER-BY clause.
A query can contain more than one sub-query.
Types of Sub-Query
Single-row sub-query, where the sub-query returns only one row.
Multiple-row sub-query, where the sub-query returns multiple rows,. and
Multiple column sub-query, where the sub-query 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 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.
What are 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 non clustered 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 non
clustered 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 indexes
A clustered index
A clustered index and many nonclustered indexes
A nonclustered index
Many nonclustered indexes
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 are the advantages of using Stored Procedures?
Stored procedure can reduced network traffic and latency, boosting application performance.
Stored procedure execution plans can be reused, staying cached in SQL Server's memory, reducing server overhead.
Stored procedures help promote code reuse.
Stored procedures can encapsulate logic. You can change stored procedure code without affecting clients.
Stored procedures provide better security to your data.
What is UNIQUE KEY constraint?
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.
What is FOREIGN KEY?
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.
What is CHECK Constraint?
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.
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 and can be used this as the Disaster Recovery plan. The key feature of log shipping is that it will automatically backup transaction logs
throughout the day and automatically restore them on the standby server at defined interval.
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. BULK INSERT command helps to
import a data file into a database table or view in a user-specified format.
What is SQL Server?
SQL Server is DBMS system provided by Microsoft. Sometimes it is mistakenly referred as SQL but both are totally different, as SQL is a language whereas SQL Server is a
Microsoft product that supports SQL.
What is collation sensitivity? Explain different types.
Collation sensitivity is used to define the rules for sorting and comparing the strings of character data.
Different types of collation sensitivity:
Case sensitivity
Accent sensitivity
Kana sensitivity
Width sensitivity
What are the authentication modes in SQL Server? How can it be changed?
SQL Server supports two authentication modes:
Window authentication mode and mixed mode.
Window authentication mode: This authentication mode is used to connect through a Microsoft NT 4.0 or window 2000 user account.
Mixed mode: It is used to connect with the instance of SQL Server using window authentication or SQL Server authentication.
What is DBCC command and why is it used?
DBCC stands for database consistency checker. This command is used to check the consistency of the database. For example:
DBCC CHECKDB:
It makes sure that table in the database and the indexes are correctly linked.
DBCC CHECKALLOC:
It checks all pages in the database and make sure that all are correctly allocated.
DBCC CHECKFILEGROUP:
It checks all table file group for any damage.
What is scheduled jobs or scheduled tasks?
Scheduled tasks let you manage the tasks in an automated manner that runs on regular or predictable cycles. You can schedule administrative tasks and also determine the order in
which tasks will run.
What is SQL Server Profiler?
Microsoft SQL Server profiler is a graphical user interface that allows system administrators to monitor events of database engine.
You can do the following things with a SQL Server Profiler -
You can create a trace.
You can watch the trace results when the trace runs.
You can store the trace results in a table.
If it is necessary, you can start, stop, pause and modify the trace results.
What is SQL Server agent?
SQL Server agent is a component of Microsoft SQL Server. It is a background tool of Microsoft SQL Server so it runs continuously in the background as a window service. SQL
Server agent allows the database administrator to handles automated tasks and schedules jobs. It runs a window service so can start automatically when the system boots or you
can start it manually.
What are the basic functions for master, msdb, model, tempdb and resource databases?
The master database holds information for all databases located on the SQL Server instance and is theglue that holds the engine together. Because SQL Server cannot start without
a functioning masterdatabase, 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.
The resoure Database is a read-only database that contains all the system objects that are included with SQL Server. SQL Server system objects, such as sys.objects, are
physically persisted in the Resource database, but they logically appear in the sys schema of every database. The Resource database does not contain user data or user metadata.
What is Service Broker?
Service Broker is a message-queuing technology in SQL Server that allows developers to integrate SQL Server fully into distributed applications. Service Broker is feature which
provides facility to SQL Server to send an asynchronous, transactional message. it allows a database to send a message to another database without waiting for the response, so
the application will continue to function if the remote database is temporarily unavailable.
Where SQL server user names and passwords are stored in SQL server?
They get stored in System Catalog Views sys.server_principals and sys.sql_logins.
What is Policy Management?
Policy Management in SQL SERVER 2008 allows you to define and enforce policies for configuring and managing SQL Server across the enterprise. Policy-Based Management is
configured in SQL Server Management Studio (SSMS). Navigate to the Object Explorer and expand the Management node and the Policy Management node; you will see the
Policies, Conditions, and Facets nodes.
What is Replication and Database Mirroring?
Database mirroring can be used with replication to provide availability for the publication database. Database mirroring involves two copies of a single database that typically reside
on different computers. At any given time, only one copy of the database is currently available to clients which are known as the principal database. Updates made by clients to the
principal database are applied on the other copy of the database, known as the mirror database. Mirroring involves applying the transaction log from every insertion, update, or
deletion made on the principal database onto the mirror database.
What are Sparse Columns?
A sparse column is another tool used to reduce the amount of physical storage used in a database. They are the ordinary columns that have an optimized storage for null values.
Sparse columns reduce the space requirements for null values at the cost of more overhead to retrieve nonnull values.
What does TOP Operator Do?
The TOP operator is used to specify the number of rows to be returned by a query. The TOP operator has new addition in SQL SERVER 2008 that it accepts variables as well as
literal values and can be used with INSERT, UPDATE, and DELETES statements.
What is CTE?
CTE is an abbreviation Common Table Expression. A Common Table Expression (CTE) is an expression that can be thought of as a temporary result set which is defined within
the execution of a single SQL statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query.
What is MERGE Statement?
MERGE is a new feature that provides an efficient way to perform multiple DML operations. In previous versions of SQL Server, we had to write separate statements to INSERT,
UPDATE, or DELETE data based on certain conditions, but now, using MERGE statement we can include the logic of such data modifications in one statement that even checks
when the data is matched then just update it and when unmatched then insert it. One of the most important advantages of MERGE statement is all the data is read and processed
only once.
What is Filtered Index?
Filtered Index is used to index a portion of rows in a table that means it applies filter on INDEX which improves query performance, reduce index maintenance costs, and reduce
index storage costs compared with full-table indexes. When we see an Index created with some where clause then that is actually a FILTERED INDEX.
Which are new data types introduced in SQL SERVER 2008?
The GEOMETRY Type: The GEOMETRY data type is a system .NET common language runtime (CLR) data type in SQL Server. This type represents data in a two-dimensional
Euclidean coordinate system.
The GEOGRAPHY Type: The GEOGRAPHY datatype’s functions are the same as with GEOMETRY. The difference between the two is that when you specify GEOGRAPHY,
you are usually specifying points in terms of latitude and longitude.
New Date and Time Datatypes: SQL Server 2008 introduces four new datatypes related to date and time: DATE, TIME, DATETIMEOFFSET, and DATETIME2.
DATE: The new DATE type just stores the date itself. It is based on the Gregorian calendar and handles years from 1 to 9999.
TIME: The new TIME (n) type stores time with a range of 00:00:00.0000000 through 23:59:59.9999999. The precision is allowed with this type. TIME supports seconds down to
100 nanoseconds. The n in TIME (n) defines this level of fractional second precision, from 0 to 7 digits of precision.
The DATETIMEOFFSET Type: DATETIMEOFFSET (n) is the time-zone-aware version of a datetime datatype. The name will appear less odd when you consider what it really
is: a date + a time + a time-zone offset. The offset is based on how far behind or ahead you are from Coordinated Universal Time (UTC) time.
The DATETIME2 Type: It is an extension of the datetime type in earlier versions of SQL Server. This new datatype has a date range covering dates from January 1 of year 1
through December 31 of year 9999. This is a definite improvement over the 1753 lower boundary of the datetime datatype. DATETIME2 not only includes the larger date range,
but also has a timestamp and the same fractional precision that TIME type provides
What are the Advantages of using CTE?
Using CTE improves the readability and makes maintenance of complex queries easy.
The query can be divided into separate, simple, logical building blocks which can be then used to build more complex CTEs until final result set is generated.
CTE can be defined in functions, stored procedures, triggers or even views.
After a CTE is defined, it can be used as a Table or a View and can SELECT, INSERT, UPDATE or DELETE Data.
What is CLR?
In SQL Server 2008, SQL Server objects such as user-defined functions can be created using such CLR languages. This CLR language support extends not only to user-defined
functions, but also to stored procedures and triggers. You can develop such CLR add-ons to SQL Server using Visual Studio 2008.
What are synonyms?
Synonyms give you the ability to provide alternate names for database objects. You can alias object names; for example, using the Employee table as Emp. You can also shorten
names. This is especially useful when dealing with three and four part names; for example, shortening server.database.owner.object to object.
What is LINQ?
Language Integrated Query (LINQ) adds the ability to query objects using .NET languages. The LINQ to SQL object/relational mapping (O/RM) framework provides the following
basic features:
Tools to create classes (usually called entities) mapped to database tables
Compatibility with LINQ's standard query operations
The DataContext class, with features such as entity record monitoring, automatic SQL statement generation, record concurrency detection, and much more
What is Isolation Levels?
Transactions specify an isolation level that defines the degree to which one transaction must be isolated from resource or data modifications made by other transactions. Isolation
levels are described in terms of which concurrency side-effects, such as dirty reads or phantom reads, are allowed.
Transaction isolation levels control:
Whether locks are taken when data is read, and what type of locks are requested.
How long the read locks are held.
Whether a read operation referencing rows modified by another transaction:
Blocks until the exclusive lock on the row is freed.
Retrieves the committed version of the row that existed at the time the statement or transaction started.
Reads the uncommitted data modification.
What is use of EXCEPT Clause?
EXCEPT clause is similar to MINUS operation in Oracle. The EXCEPT query and MINUS query returns all rows in the first query that are not returned in the second query. Each
SQL statement within the EXCEPT query and MINUS query must have the same number of fields in the result sets with similar data types.
How would you handle error in SQL SERVER 2008?
SQL Server now supports the use of TRY...CATCH con handling. TRY...CATCH lets us build error handling at the level we need, in the way w to, by setting a region where if any
error occurs, it will break out of the region and head to an error handler.
The basic structure is as follows:
BEGIN TRY
stmts..
END TRY
BEGIN CATCH
stmts..
END CATCH
What is RAISEERROR?
RaiseError generates an error message and initiates error processing for the session. RAISERROR can either reference a user-defined message stored in the sys.messages catalog
view or build a message dynamically. The message is returned as a server error message to the calling application or to an associated CATCH block of a TRY | CATCH construct.
How to rebuild Master Database?
Master database is system database and it contains information about running server's configuration. When SQL Server 2005 is installed it usually creates master, model, msdb,
tempdb resource and distribution system database by default. Only Master database is th one which is absolutely must have database. Without Master database SQL Server cannot
be started. This is the reason it is extremely important to backup Master database.
To rebuild the Master database, Run Setup.exe, verify, and repair a SQL Server instance, and rebuild the system databases. This procedure is most often used to rebuild the master
database for a corrupted installation of SQL Server.
What is XML Datatype?
The xml data type lets you store XML documents and fragments in a SQL Server database. An XML fragment is an XML instance that is missing a single top-level element. You
can create columns and variables of the xml type and store XML instances in them. The xml data type and associated methods help integrate XML into the relational framework of
S Server.
What is Data Compression?
In SQL SERVE 2008 Data Compression comes in two flavors:
Row Compression: Row compression changes the format of physical storage of data. It minimize the metadata (column information, length, offsets etc) associated with each
record. Numeric data types and fixed length strings are stored in variable-length storage format, just like Varchar.
Page Compression: Page compression allows common data to be shared between rows for a given page. Its uses the following techniques to compress data:
Row compression.
Prefix Compression. For every column in a page duplicate prefixes are identified. These prefixes are saved in compression information headers (CI) which resides after page
header. A reference number is assigned to these prefixes and that reference number is replaced where ever those prefixes are being used.
Dictionary Compression: Dictionary compression searches for duplicate values throughout the page and stores them in CI. The main difference between prefix and dictionary
compression is that prefix is only restricted to one column while dictionary is applicable to the complete page.
What is Catalog Views?
Catalog views return information that is used by the SQL Server Database Engine. Catalog Views are the most general interface to the catalog metadata and provide the most
efficient way to obtain, transform, and present customized forms of this information. All user- available catalog metadata is exposed through catalog views.
What is PIVOT and UNPIVOT?
A Pivot Table can automatically sort, count, and total the data stored in one table or spreadsheet and create a second table displaying the summarized data. The PIVOT operator
turns the values of a specified column into column names, effectively rotating a table.
UNPIVOT table is reverse of PIVOT Table.
What is Dirty Read ?
A dirty read occurs when two operations say, read and write occurs together giving the incorrect or unedited data. Suppose, A has changed a row, but has not committed the
changes. B reads the uncommitted data but his view of the data may be wrong so that is Dirty Read.
What is Aggregate Functions?
Aggregate functions perform a calculation on a set of values and return a single value. Aggregate functions ignore NULL values except COUNT function. HAVING clause is used,
along with GROUP BY, for filtering query using aggregate values.
Following functions are aggregate functions.
AVG, MIN CHECKSUM_AGG, SUM, COUNT, STDEV, COUNT_BIG, STDEVP, GROUPING, VAR, MAX. VARP
What do you mean by Table Sample?
TABLESAMPLE allows you to extract a sampling of rows from a table in the FROM clause. The rows retrieved are random and they are not in any order. This sampling can be
based on a percentage of number of rows. You can use TABLESAMPLE when only a sampling of rows is necessary for the application instead of a full result set.
What is the difference between UNION and UNION ALL?
UNION The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected
columns need to be of the same data type. With UNION, only distinct values are selected.
UNION ALL The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines
them into a table.
What is B-Tree?
The database server uses a B-tree structure to organize index information. B-Tree generally has following types of index pages or nodes:
root node: A root node contains node pointers to branch nodes which can be only one.
branch node: A branch node contains pointers to leaf nodes or other branch nodes which can be two or more.
leaf nodes: A leaf node contains index items and orizantal pointers to other leaf nodes which can be many