Difference between UNION and UNION ALL in SQL Server

Vipendra Verma

14y1.9k

Hi friends

I want to know what is the difference between UNION and UNION ALL in SQL Server ?

Thank you.

6 Replies

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Govardhan P6y ago
In simple terms we can say that UNION is to fetch everything without duplicates.
UNION ALL is to fetch everything including duplicates.
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Mageshwaran R6y ago
Hi vipendra verma,
Union:
1.Uninon is two table get common or distinct value only.
2.Union command is select related information from two tables,like join command.
Union All:
uninon All is two table get all values.
The main differnce between union and union all is that union all will not eliminate duplicate rows,but union all is eliminate duplicate rows.
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Sourav Mukherjee7y ago
Union all will not remove the duplicate rows whereas it just pulls all rows from all tables matchng with your query specifics and merges them into a table. On the other hand, a UNION statement effectively works just like a SELECT DISTINCT on the results set.
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Aniket Narvankar7y ago
1.Union All Selects Duplicate Records,while Union Selects Distinct Records
2.Union All is faster than Union.Becauses Union orders the records while selecting them.
3.Both in case of Union and Union All the type and number of columns must same in both sides of select statement.Also if you are using order by clause you should use with last select statement both in case of union and union all.
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Satyapriya Nayak14y ago
Hi Vipendra,

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

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

Thanks
+2
14y ago
When you use union statement, it will not return duplicate values.
When you use union-all statement, it will return duplicate values also.

http://blog.sqlauthority.com/2009/03/11/sql-server-difference-between-union-vs-union-all-optimal-performance-comparison/

http://blog.sqlauthority.com/2007/03/10/sql-server-union-vs-union-all-which-is-better-for-performance/
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