一条sql语句完成MySQL去重留一

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• mysql 安装、sql语法 
• 一条sql语句完成MySQL去重留一
• mysql 事务、索引、锁、分区/分表、sql优化、查询优化
• mysql 安装配置使用、sql语法
• MySQL主键、创建索引、UNION 和 UNION ALL

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DELETE 
    mygame
FROM 
    mygame, 
(
SELECT
   min(id) id,
   game,
   userid,
   create_datatime
FROM
   mygame
GROUP BY
   game,
   userid,
HAVING
   count(*) > 1
) t2
WHERE
    mygame.game = t2.game
and mygame.userid = t2.userid
and mygame.id > t2.id

第一步： 查询出重复记录形成一个集合（临时表t2），集合里是每种重复记录的最小ID
SELECT
   min(id) id,
   game,
   userid,
   create_datatime
FROM
   mygame
GROUP BY
   game,
   userid,
HAVING
   count(*) > 1
) t2


第二步：关联 判断重复基准的字段，根据条件，删除原表中id大于t2中id的记录
DELETE 
    mygame
FROM 
    mygame, t2
WHERE
    mygame.game = t2.game
and mygame.userid = t2.userid
and mygame.id > t2.id

在使用mysql时，有时需要查询出某个字段不重复的记录，虽然mysql提供 有distinct这个关键字来过滤掉多余的重复记录只保留一条，
但往往只用它来返回不重复记录的条数，而不是用它来返回不重记录的所有值。

DELETE consum_record
FROM
 consum_record, 
 (
  SELECT
   min(id) id,
   user_id,
   monetary,
   consume_time
  FROM
   consum_record
  GROUP BY
   user_id,
   monetary,
   consume_time
  HAVING
   count(*) > 1
 ) t2
WHERE
 consum_record.user_id = t2.user_id 
 and consum_record.monetary = t2.monetary
 and consum_record.consume_time = t2.consume_time
AND consum_record.id > t2.id;

上面这条sql语句，仔细看一下，揣摩出思路也不难，大概也分为3步来理解：
(SELECT min(id) id, user_id, monetary, consume_time FROM consum_record GROUP BY user_id, monetary, consume_time HAVING count(*) > 1 ) t2
 查询出重复记录形成一个集合（临时表t2），集合里是每种重复记录的最小ID

consum_record.user_id = t2.user_id and consum_record.monetary = t2.monetary and consum_record.consume_time = t2.consume_time 
关联 判断重复基准的字段，根据条件，删除原表中id大于t2中id的记录