Difference between UNION and UNIONALL

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

Table Internet_Sales

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:

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

Table Internet_Sales

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:

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