mysql - 如何删除 'duplicate'记录？

Question

我有一个表包含一些损坏的记录，因为我忘记了 为两列添加一个 UNIQUE 索引。 请查看下表的示例:

+----+-------------+--------+------------+
| id | uuid        | object | project_id |
+----+-------------+--------+------------+
| 1  | 73621000001 | screw  | 1          |
| 2  | 73621000002 | screw  | 1          |
| 3  | 73621000003 | screw  | 1          |
| 4  | 73621000004 | tube   | 1          |
| 5  | 73621000005 | plate  | 2          |
| 6  | 73621000006 | plate  | 2          |
| 7  | 73621000007 | plate  | 2          |
| 8  | 73621000008 | plate  | 2          |
| 9  | 73621000009 | plate  | 2          |
| 10 | 73621000010 | gear   | 4          |
| 11 | 73621000011 | gear   | 4          |
+----+-------------+--------+------------+

如您所见，有一些object-project_id-组合出现多次，但具有不同的uuid。 我想删除所有重复记录，但保留具有最高 uuid 的记录。 结果表应该是这样的:

+----+-------------+--------+------------+
| id | uuid        | object | project_id |
+----+-------------+--------+------------+
| 3  | 73621000003 | screw  | 1          |
| 4  | 73621000004 | tube   | 1          |
| 9  | 73621000009 | plate  | 2          |
| 11 | 73621000011 | gear   | 4          |
+----+-------------+--------+------------+

我可以使用以下查询查看哪些对象有重复项:

SELECT uuid, object, project_id, COUNT(*)
FROM uuid_object_mapping
GROUP BY object, project_id
HAVING COUNT(*) > 1;

我可以使用此查询获取“干净”的表:

SELECT MAX(uuid) as uuid, object, project_id
FROM uuid_object_mapping
GROUP BY object, project_id;

我可以使用以下命令验证“干净”表不包含重复项

SELECT uuid, object, project_id, COUNT(*)
FROM (
    SELECT MAX(uuid) as uuid, object_name, project_id
    FROM uuid_object_mapping
    GROUP BY object_name, project_id
) AS clean
GROUP BY object_name, project_id
HAVING COUNT(*) > 1;

但是我怎样才能删除“干净”表中没有的所有内容呢？

Answer 1

在 MySQL 中，您可以使用 join，但需要注意 NULL 值:

delete om
    from uuid_object_mapping om join
         (select MAX(uuid) as uuid, object, project_id
          from uuid_object_mapping 
          group by object, project_id
         ) omkeep
         on omkeep.object = om.object and
            omkeep.project_id <=> om.project_id
    where om.uuid <> omkeep.uuid;

NULL 值似乎已经消失，因此您可以使用此 on 子句:

         on omkeep.object = om.object and
            omkeep.project_id = om.project_id