SQL SERVER Interview Questions

1.What is DTS in SQL Server ?
Data Transformation Services is used to transfer the data from one source to our required destination. Considering am having some data in sql server and I need to transfer the data to Excel destination. Its highly possible with dialogue based tool called Data Transformation services. More customization can be achieved using SSIS. A specialized tool used to do such migration works.

2.What is the significance of NULL value and why should we avoid permitting null values?
Null means no entry has been made. It implies that the value is either unknown or undefined.We should avoid permitting null values because Column with NULL values can't have PRIMARY KEY constraints. Certain calculations can be inaccurate if NULL columns are involved.

3.Difference between primary key and Unique key?
Both constraints will share a common property called uniqueness. The data in the column should be unique. The basic difference is,
· Primary key won’t allow null value. Whereas, unique key will accept null value but only one null value.
· On creating primary key, it will automatically format the data inturn creates clustered index on the table. Whereas, this characteristics is not associated with unique key.
· Only one primary key can be created for the table. Any number of Unique key can be created for the table.

4.What is a stored procedure:
Stored procedures are precompiled T-SQL statements combined to perform a single task of several tasks. Its basically like a Macro so when you invoke the Stored procedure, you actually run a set of statements. As, its precompiled statement, execution of Stored procedure is compatatively high when compared to an ordinary T-SQL statement.

5.What is the difference between UNION ALL Statement and UNION ?
The main difference between UNION ALL statement and UNION is UNION All statement is much faster than UNION,the reason behind this is that because UNION ALL statement does not look for duplicate rows, but on the other hand UNION statement does look for duplicate rows, whether or not they exist.

6.Example for Stored Procedure?
They are three kinds of stored procedures,1.System stored procedure – Start with sp_2. User defined stored procedure – SP created by the user.3. Extended stored procedure – SP used to invoke a process in the external systems.Example for system stored proceduresp_helpdb - Database and its propertiessp_who2 – Gives details about the current user connected to your system. sp_renamedb – Enable you to rename your database

7.What is a trigger?

Triggers are precompiled statements similar to Stored Procedure. It will automatically invoke for a particular operation. Triggers are basically used to implement business rules.

8.What is a view?
If we have several tables in a db and we want to view only specific columns from specific tables we can go for views. It would also suffice the needs of security some times allowing specfic users to see only specific columns based on the permission that we can configure on the view. Views also reduce the effort that is required for writing queries to access specific columns every time.

9.What is an Index?
When queries are run against a db, an index on that db basically helps in the way the data is sorted to process the query for faster and data retrievals are much faster when we have an index.

10.What are the types of indexes available with SQL Server?

There are basically two types of indexes that we use with the SQL ServerClustered -

1. It will format the entire table, inturn physically sort the table.

2. Only one clustered index can be created for a table.

3. Data will be located in the leaf level.

4. By default, primary key will create clustered index on the table.

Non-Clustered Index

1. It wont touch the structure of the table.

2. It forms an index table as reference to the exact data.

3. A reference to the data will be located in the leaf level.

4. For a table, we can create 249 non clustered index.

11.Extent Vs Page?

Pages are low level unit to store the exact data in sql server. Basically, the data will be stored in the mdf, ldf, ndf files. Inturn, pages are logical units available in sql server.The size of the page is 8KB.

Eight consecutive pages will form an extent 8 * 8KB = 64KB.

Thus I/O level operation will be happening at pages level.The pages will hold a template information at the start of each page (header of the page).

They are,

1. page number,

2. page type,

3. the amount of free space on the page,

4. the allocation unit ID of the object that owns the page.

Extents will be classifed into two types,

1. Uniform extents

2. Mixed extents

Uniform Extents:It occupied or used by a single object. Inturn, a single object will hold the entire 8 pages.Mixed

Extents:Mulitple objects will use the same extent. SQL Server will allow a max of eight objects to use a shared extent.

Property of SQL Server :Initally if an object is created, sql server will allocate the object to the mixed extent and once if the size reaches 8 pages and more... immediately, a new uniform extent will be provided for that particular object.

Herecomes, our fragmentation and reindexing concepts.

some information of sql server:

Maximum Number of tables : No restrictions (Until your harddisk size you can create tables)
Maximum bytes per row in the table : 8060 bytes
Maximum tables in a select statment - 256
Maxiumu references per table - 253
Maximum rows in the table - Unlimited (Depends on the storage)
Maximum columns per base table : 1024
Maximum columns per insert statment : 1024
Maximum columns per Select statment : 1024 * 4 = 4096
Bytes per Group by or Order by = 8000

Index:
Maximum clustered index per table - 1
Maximum Non-clustered index per table - 249
Maximum Columns in an index (composite index) - 16

Keys:
Maximum Columns in a Primary key - 16
Maximum Columns in a Foreign key - 16

Objects:
Nested Stored procedure levels- 32
Nested triggers levels- 32
Parameters per stored procedure - 2100
Parameters per User defined functions - 2100
Identifier length - 16

Database Level Details:
Number of instances for a server - 50
Files per database - 32767
Filegroups per database - 32767
Databases per instance of SQL Server - 32767

12.MSDE vs SQL Server :
MSDE is the downgrade version of SQL Server, the same code with initial release of SQL Server was named as MSDE (Microsoft Desktop Enginer).

13.Extent Vs Page:
Pages are 8 KB size smallest unit to store the data. Inturn, 8 Pages will form the extent.

14.Delete VS Truncate:
Delete will delete row by row in the table. The syntax is,
delete from tablename
We can give where conditions for deleting the data. 
Each delete will be logged in the log file.
Its a DML statement

Trucate will deallocate the reference in the pages instead of deleting the data.
Its DDL statement. The syntax is,
Truncate table tablename
We can provide truncate with tables having foreign key relationships. (Because it wont do any cascade delete on the table).
It wont log any information in the log file.
Delete or truncate the data at a stretch and wont do any row by row manipulation.

15.What do you mean by acid property?

Atomicity : The value should be atomic.

Consistency : The data in the database should be consistent. It indicates, if we are maintaining data in two different places. Those data should resemble. If it differs, then it will create confusions. Am I right?

Isolation : Transaction should be isolated and it should not affect the other transactions. If the transactions occured on the same data. Appropriate locks should be posted before starting the transactions.

Durability: The data should be stable.