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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]
[SQL Statement 2];
Say we have the following two tables,
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
SELECT Txn_Date FROM Internet_Sales;
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.