UNION
puts lines from queries after each other, while JOIN
makes a cartesian product and subsets it — completely different operations. Trivial example of UNION
:
mysql> SELECT 23 AS bah -> UNION -> SELECT 45 AS bah; +-----+ | bah | +-----+ | 23 | | 45 | +-----+ 2 rows in set (0.00 sec)
similary trivial example of JOIN
:
mysql> SELECT * FROM -> (SELECT 23 AS bah) AS foo -> JOIN -> (SELECT 45 AS bah) AS bar -> ON (33=33); +-----+-----+ | foo | bar | +-----+-----+ | 23 | 45 | +-----+-----+ 1 row in set (0.01 sec)
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