SQL UNION

SQL Union is used to consolidate the records in the two table into single result and both columns list and names should be sale to combine the records.

It will not filter the duplicate records. It combines the rows rather than columns.

For example, have two tables.

SELECT id,name,address FROM Employee1
UNION
SELECT id,name,address FROM Employee2

It will combine the two table results.

Hi Arun,

UNION

The purpose of the SQL UNION query is to combine the results of two queries together. In this respect, UNION is somewhat similar to JOIN in that they are both used to related information from multiple tables. One restriction of UNION is that all corresponding columns need to be of the same data type. Also, when using UNION, only distinct values are selected (similar to SELECT DISTINCT).

The syntax is as follows:

[SQL Statement 1]
UNION
[SQL Statement 2]

Say we have the following two tables,

Table Store_Information

store_name	Sales	Date
Los Angeles	$1500	Jan-05-1999
San Diego	$250	Jan-07-1999
Los Angeles	$300	Jan-08-1999
Boston	$700	Jan-08-1999

Table Internet_Sales

Date	Sales
Jan-07-1999	$250
Jan-10-1999	$535
Jan-11-1999	$320
Jan-12-1999	$750

and we want to find out all the dates where there is a sales transaction. To do so, we use the following SQL statement:

SELECT Date FROM Store_Information
UNION
SELECT Date FROM Internet_Sales

Result:

Date

Jan-05-1999

Jan-07-1999

Jan-08-1999

Jan-10-1999

Jan-11-1999

Jan-12-1999

UNION ALL

The purpose of the SQL UNION ALL command is also to combine the results of two queries together. The difference between UNION ALL and UNION is that, while UNION only selects distinct values, UNION ALL selects all values.

The syntax for UNION ALL is as follows:

[SQL Statement 1]
UNION ALL
[SQL Statement 2]

Let's use the same example as the previous section to illustrate the difference. Assume that we have the following two tables,

Table Store_Information

store_name	Sales	Date
Los Angeles	$1500	Jan-05-1999
San Diego	$250	Jan-07-1999
Los Angeles	$300	Jan-08-1999
Boston	$700	Jan-08-1999

Table Internet_Sales

Date	Sales
Jan-07-1999	$250
Jan-10-1999	$535
Jan-11-1999	$320
Jan-12-1999	$750

and we want to find out all the dates where there is a sales transaction at a store as well as all the dates where there is a sale over the internet. To do so, we use the following SQL statement:

SELECT Date FROM Store_Information
UNION ALL
SELECT Date FROM Internet_Sales

Result:

Date

Jan-05-1999

Jan-07-1999

Jan-08-1999

Jan-08-1999

Jan-07-1999

Jan-10-1999

Jan-11-1999

Jan-12-1999

UNION  is used to select information from two tables.But it selects only distinct records from both the table. , while UNION ALL selects all records from both the tables.



Please refer the below links

http://www.1keydata.com/sql/sqlunion.html

http://www.1keydata.com/sql/sqlunionall.html

Thanks

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.

http://blog.sqlauthority.com/2008/10/15/sql-server-introduction-and-example-of-union-and-union-all/