问题描述
如何从 MySQL 表中删除所有重复数据?
How would I delete all duplicate data from a MySQL Table?
例如，使用以下数据:
SELECT * FROM names;
+----+--------+
| id | name |
+----+--------+
| 1 | google |
| 2 | yahoo |
| 3 | msn |
| 4 | google |
| 5 | google |
| 6 | yahoo |
+----+--------+
如果是 SELECT 查询，我会使用 SELECT DISTINCT name FROM names;.
I would use SELECT DISTINCT name FROM names; if it were a SELECT query.
我将如何使用 DELETE 执行此操作，以仅删除重复项并仅保留每个记录?
How would I do this with DELETE to only remove duplicates and keep just one record of each?
推荐答案
编辑警告:此解决方案的计算效率低下，并且可能会导致大型表的连接中断.
注意 - 您需要首先在表的测试副本上执行此操作！
NB - You need to do this first on a test copy of your table!
当我这样做的时候，我发现除非我也包括了 AND n1.id <>n2.id，它删除了表中的每一行.
When I did it, I found that unless I also included AND n1.id <> n2.id, it deleted every row in the table.
如果要保留
id值最低的行:
DELETE n1 FROM names n1, names n2 WHERE n1.id > n2.id AND n1.name = n2.name
如果要保留 id 值最高的行:
DELETE n1 FROM names n1, names n2 WHERE n1.id < n2.id AND n1.name = n2.name
我在 MySQL 5.1 中使用了这种方法
I used this method in MySQL 5.1
不确定其他版本.
更新:由于人们在谷歌上搜索删除重复项到此结束
虽然 OP 的问题是关于 DELETE，但请注意，使用 INSERT 和 DISTINCT 会快得多.对于一个有 800 万行的数据库，下面的查询耗时 13 分钟，而使用 DELETE 则耗时 2 个多小时，但仍未完成.
Update: Since people Googling for removing duplicates end up here
Although the OP's question is about DELETE, please be advised that using INSERT and DISTINCT is much faster. For a database with 8 million rows, the below query took 13 minutes, while using DELETE, it took more than 2 hours and yet didn't complete.
INSERT INTO tempTableName(cellId,attributeId,entityRowId,value)
SELECT DISTINCT cellId,attributeId,entityRowId,value
FROM tableName;
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