MySQL多表、左链接、嵌套查询

多表查询

注意限定范围以及where限定语句

mysql> select user.name,content.tittle from user,content where user.id=content.uid;+-------+-----------+| name  | tittle    |+-------+-----------+| user1 | aaaaaaaaa || user2 | ab        || user2 | abcd      || user3 | eeeee     || user3 | eeeee     || user4 | jgnsfkn   || user4 | jgnsfkn   || user4 | jgnsfkn   |+-------+-----------+8 rows in set (0.00 sec)

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左链接查询

mysql> select user.name,content.tittle from (user left join content on user.id=content.uid);+-------+-----------+| name  | tittle    |+-------+-----------+| user1 | aaaaaaaaa || user2 | ab        || user2 | abcd      || user3 | eeeee     || user3 | eeeee     || user4 | jgnsfkn   || user4 | jgnsfkn   || user4 | jgnsfkn   || user5 | NULL      |+-------+-----------+9 rows in set (0.00 sec)

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嵌套查询

mysql> select name from user where id in (select uid from content);+-------+| name  |+-------+| user1 || user2 || user3 || user4 |+-------+4 rows in set (0.00 sec)//即使id有索引也已经失效