MySQL查询重复数据

 

假设有一个用户表 user，数据如下：

1、查询表中 uid 重复的数据

SELECT
  id,
  uid,
  name
FROM USER
WHERE uid IN (SELECT
    uid
  FROM USER
  GROUP BY uid
  HAVING COUNT(uid) > 1);

2、查询表中重复数据，排除最小id

SELECT
  id,
  uid,
  name
FROM user
WHERE uid IN (SELECT
    uid
  FROM user
  GROUP BY uid
  HAVING COUNT(uid) > 1)
AND id NOT IN (SELECT
    MIN(id)
  FROM user
  GROUP BY uid
  HAVING COUNT(uid) > 1);

3、删除表中重复数据，如果是重复数据，则保留id最小的一条

DELETE
  FROM USER
WHERE id IN (SELECT
      u.id
    FROM (SELECT
        id
      FROM USER
      WHERE uid IN (SELECT
          uid
        FROM USER
        GROUP BY uid
        HAVING COUNT(uid) > 1)
      AND id NOT IN (SELECT
          MIN(id)
        FROM USER
        GROUP BY uid
        HAVING COUNT(uid) > 1)) AS u);

4、遇到的问题：

一开始直接使用以下语句删除，报错：You can’t specify target table ‘user’ for update in FROM clause

 

DELETE
  FROM USER
WHERE id IN (SELECT
      id
    FROM USER
    WHERE uid IN (SELECT
        uid
      FROM USER
      GROUP BY uid
      HAVING COUNT(uid) > 1)
    AND id NOT IN (SELECT
        MIN(id)
      FROM USER
      GROUP BY uid
      HAVING COUNT(uid) > 1));

 

查资料后得知：
因为在 MYSQL 里，不能先 select 一个表的记录，在按此条件进行更新和删除同一个表的记录。
解决办法：
将 select 得到的结果，再通过中间表 select 一遍。
SQL如下：

 

DELETE
  FROM USER
WHERE id IN (SELECT
      u.id
    FROM (SELECT
        id
      FROM USER
      WHERE uid IN (SELECT
          uid
        FROM USER
        GROUP BY uid
        HAVING COUNT(uid) > 1)
      AND id NOT IN (SELECT
          MIN(id)
        FROM USER
        GROUP BY uid
        HAVING COUNT(uid) > 1)) AS u);