mysql日常小总结(其实就今天)

联表查询:

SELECT t1.user_Name  FROM t_user AS t1   ,  t_comment AS t2 WHERE t2.user_id=t1.id

结果如图:

加上GROUP BY user_name（SELECT t1.user_Name  FROM t_user AS t1   ,  t_comment AS t2 WHERE t2.user_id=t1.id GROUP BY user_name）

如图：

GROUP BY的作用很明显了吧

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更新：

UPDATE t_comment SET state =1

UPDATE t_article SET index_state = 0  WHERE id <= 50

删除表的某一列：

ALTER TABLE t_article DROP COLUMN index_state;