mysql 如何去重

 

sql代碼：

去除重複：

delete  FROM  test where id NOT in(  select id FROM tmp  )


create table tmp (select id FROM test GROUP BY `name` )

tmp是創建的臨時表，不需要提前創建，直接寫接好，

爲什麼要創建臨時表，看下面：

直接寫會報錯 ：  

delete  FROM  test where id NOT in(  select id FROM test GROUP BY `name`  )

[SQL]

delete  FROM  test where id NOT in(  select id FROM test GROUP BY `name`  )

[Err] 1093 - You can't specify target table 'test' for update in FROM clause

 

要先運行create table tmp (select id FROM test GROUP BY `name` ) 

先創建臨時表，才能查詢

 

去除重複保留最大ID：

DELETE a FROM  test a JOIN 
( SELECT  `name`, count(*), MAX(id) AS id FROM test GROUP BY name HAVING COUNT(*) > 1) b ON a.name = b.name WHERE a.id < b.id