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SQL Union

The purpose of the SQL UNION command 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]

Let's 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. 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

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

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