mysql平常小總結(其實就今天)

聯表查詢:

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

結果如圖:

 

 

 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）

如圖：

 

 GROUP BY的做用很明顯了吧

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更新：

UPDATE t_comment SET state =1

UPDATE t_article SET index_state = 0  WHERE id <= 50

 

 

刪除表的某一列：

ALTER TABLE t_article DROP COLUMN index_state;