MySQL查詢重復數據

 

假設有一個用戶表 user，數據如下：

1、查詢表中 uid 重復的數據

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

2、查詢表中重復數據，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、刪除表中重復數據，如果是重復數據，則保留id最小的一條

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、遇到的問題：

一開始直接使用以下語句刪除，報錯：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));

 

查資料后得知：
因為在 MYSQL 里，不能先 select 一個表的記錄，在按此條件進行更新和刪除同一個表的記錄。
解決辦法：
將 select 得到的結果，再通過中間表 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);