MySQL UNION Operator

Optimize MySQL database

Using MySQL UNION Operator you can combine two or more result set from a multiple table into a single result set.

UNION operator eliminates duplicate rows from the result set even you forgot to use DISTINCT operator. UNION ALL allows duplicate rows remain in the result.

In the corresponding SELECT statements the number of columns appears must be equal.

Example of UNION:

SELECT userID id, userName name
FROM users
UNION
SELECT orderUser id, orderName name
FROM orders

Here we combine the data from users and orders tables into a single result set.

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