mysql平常小总结(其实就今天)

联表查询:blog

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

结果如图:查询

 

 

 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)img

如图:co

 

 GROUP BY的做用很明显了吧ps

-----------------------------------------index

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;

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