如何删除重复的行并保留第一行？

Question

我犯了一个错误,我有不必要的重复.

我有一个包含4个关键字段的表格.A1,k1,k2,k3.

A1 是自动增量和主键.

组合k1,k2并且k3应该是唯一的,我必须在创建唯一索引之前删除重复的行.有些行有一个副本,有些行有很多.

SELECT CONCAT(k1, k2, k) AS dup_value
  FROM myviews
 GROUP BY dup_value
HAVING (COUNT(dup_value) > 1)

向我展示了我需要处理的重复值.但现在我不知道如何保留一个并删除每个重复集的其余部分.

Answer 1

备份您的数据,然后......

MySQL支持DELETE语句中的JOIN.如果你想保留第一个重复项:

DELETE a
  FROM MYVIEWS a
  JOIN (SELECT MIN(t.a1) AS min_a1, t.k1, t.k2, t.k3
          FROM MYVIEWS t
      GROUP BY t.k1, t.k2, t.k3
        HAVING COUNT(*) > 1) b ON b.k1 = a.k1
                              AND b.k2 = a.k2
                              AND b.k3 = a.k3
                              AND b.min_a1 != a.a1

如果你想保留最后一个重复项:

DELETE a
  FROM MYVIEWS a
  JOIN (SELECT MAX(t.a1) AS max_a1, t.k1, t.k2, t.k3
          FROM MYVIEWS t
      GROUP BY t.k1, t.k2, t.k3
        HAVING COUNT(*) > 1) b ON b.k1 = a.k1
                              AND b.k2 = a.k2
                              AND b.k3 = a.k3
                              AND b.max_a1 != a.a1