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);
+-----+-----+
| bah | bah |
+-----+-----+
| 23 | 45 |
+-----+-----+
1 row in set (0.01 sec)
http://stackoverflow.com/questions/905379/what-is-the-difference-between-join-and-union
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