SQL SERVER Interview questions and answers

What is Cursor?

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 cursor

·         Open cursor

·         Fetch row from the cursor

·         Process fetched row

·         Close cursor

·         Deallocate cursor

What is 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 thought of as views that take parameters and can be used in JOINs and other Rowset operations.

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.

What’s the difference between a primary key and a unique key?

Both primary key and unique key enforces 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 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.

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 is PRIMARY KEY?

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.

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 NOT NULL Constraint?

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.

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 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.

Can SQL Servers linked to other servers like Oracle?

SQL Server can be linked 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

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.

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 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.

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 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 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.

·         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 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 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 nin 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 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 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. 

What is XPath?

XPath uses a set of expressions to select nodes to be processed. The most common expression that you’ll use is the location path expression, which returns back a set of nodes called a node set. XPath can use both an unabbreviated and an abbreviated syntax. The following is the unabbreviated syntax for a location path:

/axisName::nodeTest[predicate]/axisName::nodeTest[predicate]

What is NOLOCK?

Using the NOLOCK query optimizer 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).

How would you handle error in SQL SERVER 2008?

SQL Server now supports the use of TRY…CATCH constructs for providing rich error handling. TRY…CATCH lets us build error handling at the level we need, in the way we need 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

<code>

END TRY

BEGIN CATCH

<code>

END CATCH

So if any error occurs in the TRY block, execution is diverted to the CATCH block, and the error can be dealt.

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 thesys.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.

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 SQL Server.

What is Data Compression?

In SQL SERVE 2008 Data Compression comes in two flavors:

·         Row Compression

·         Page Compression

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.  (Read More Here)

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.

How to copy the tables, schema and views from one SQL Server to another?

There are multiple ways to do this.

1.    “Detach Database” from one server and “Attach Database” to another server.

2.    Manually script all the objects using SSMS and run the script on new server.

3.    Use Wizard of SSMS.

How to copy data from one table to another table?

There are multiple ways to do this.

1) INSERT INTO SELECT

This method is used when table is already created in the database earlier and data is to be inserted into this table from another table. If columns listed in insert clause and select clause are same, they are not required to list them.

2) SELECT INTO

This method is used when table is not created earlier and needs to be created when data from one table is to be inserted into newly created table from another table. New table is created with same data types as selected columns.

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.

What is Filestream?

Filestream allows you to store large objects in the file system and have these files integrated within the database. It enables SQL Server based applications to store unstructured data such as documents, images, audios, videos etc. in the file system. FILESTREAM basically integrates the SQL Server Database Engine with New Technology File System (NTFS); it basically stores the data in varbinary (max) data type. Using this data type, the unstructured data is stored in the NTFS file system and the SQL Server Database Engine manages the link between the Filestream column and the actual file located in the NTFS. Using Transact SQL statements users can insert, update, delete and select the data stored in FILESTREAM enabled tables.

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 SQLCMD?

sqlcmd is enhanced version of the isql and osql and it provides way more functionality than other two options. In other words sqlcmd is better replacement of isql (which will be deprecated eventually) and osql (not included in SQL Server 2005 RTM). sqlcmd can work two modes – i) BATCH and ii) interactive modes.

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.

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 nodes: 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 horizontal pointers to other leaf nodes which can be many.

What is the purpose of the model database?

It works as Template Database for the Create Database Syntax

What is the purpose of the model database?

It works as Template Database for the Create Database Syntax

 What is the purpose of the master database?                                                                      Master database keeps the information about sql server configuration, databases users etc

What is the purpose of the tempdb database?

Tempdb database keeps the information about the temporary objects (#TableName, #Procedure). Also the sorting, DBCC operations are performed in the TempDB

What is the purpose of the USE command?

Use command is used for to select the database. For i.e Use Database Name

What is the Parse Query button used for? How does this help you?

Parse query button is used to check the SQL Query Syntax.

What is the ORDER BY used for?

Order By clause is used for sorting records in Ascending or Descending order.

What is the default order of an ORDER BY clause?

Ascending Order

In a WHERE clause, do you need to enclose a text column in quotes? Do you need to enclose a numeric column in quotes?

Enclose Text in Quotes (Yes)
Enclose Number in Quotes (NO)