SQL > Advanced SQL > 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_NameSalesTxn_Date
Los Angeles1500Jan-05-1999
San Diego250Jan-07-1999
Los Angeles300Jan-08-1999
Boston700Jan-08-1999

Table Internet_Sales

Txn_DateSales
Jan-07-1999250
Jan-10-1999535
Jan-11-1999320
Jan-12-1999750

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 Txn_Date FROM Store_Information
UNION
SELECT Txn_Date FROM Internet_Sales;

Result:

Txn_Date
Jan-05-1999
Jan-07-1999
Jan-08-1999
Jan-10-1999
Jan-11-1999
Jan-12-1999

Please note that if we type "SELECT DISTINCT Txn_Date" for either or both of the SQL statement, we will get the same result set.

Next: SQL UNION ALL




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