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  • 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);
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  • 原文地址:https://www.cnblogs.com/woods1815/p/12901717.html
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