Difference between UNION and UNION ALL ?

UNION and UNION ALL both are used to combine the results sets into one single results set, But following are the difference between the both operators.

#.	UNION	UNION ALL
1	UNION removes duplicate records from the two or more results sets	UNION ALL doen’t remove duplicate records from the two or more results sets
2	UNION  Sorts the data from the results sets	UNION ALL doesn’t Sorts the data from the results sets
3	By compare with union all, union is much less performance. Because of it must scan the result for duplicates	UNION ALL performance is more comparing with UNION

SET Operators in T-SQL

Set Operators :

It is used to combine multiple results sets into one single result set.

There are 3 set operators in T-SQL
1) UNION/UNION ALL
2) EXCEPT
3) INTERSECT

UNION/UNION ALL :
----------------------------
It is used to combine two or more result sets into one single result set by with or without duplicates
(ie, UNION doesn't allow null values, but UNION ALL allows null values)

Example:
<<ResultSet>>
UNION or UNION ALL
<<ResultSet>>


 EXCEPT:
------------
It takes the data from one result set where there is no matching in another result set.

Example:
<<ResultSet>>
EXCEPT
<<ResultSet>>

 INTERSECT:
----------------
It takes the data from the result sets which are in common

Example:
<<ResultSet>>
INTERSECT
<<ResultSet>>

How to change the Identity Column value ?

Before Change the Identity value(ProductID column) from the Product Table

 

 

-- To set the new identity value on the Product Table

DBCC CHECKIDENT ('Product', RESEED, 10)

Execution Results as follows

Checking identity information: current identity value '10', current column value '10'.
DBCC execution completed.

To insert a new record in the product table, to test the next identity value

insert into Product values ('Mat',200)

After Change the Identity value

 

How to check the current identiy value in a table ?

"DBCC CHECKIDENT" Statement is used to check the current identity value for the specified table and if requires then can modify/change the identity value on a identity column.

Syntax:
DBCC CHECKIDENT ('Table_Name' [ , { NORESEED  | { RESEED [ , new_reseed_value ] } } ] )

Example:

CREATE TABLE Product(ProductID int identity(1,1), ProductName varchar(20), Price decimal(18,6) )

INSERT INTO Product VALUES ('Book',150.10),('Pen',35.50)

-- To check the current maximum identity value in the column, execute the following statement


DBCC CHECKIDENT ('Product', NORESEED)