從表中刪除所有重複行的最有效方法是什麼？

Question

我有一個表：

| foo | bar | 
+-----+-----+ 
| a | abc | 
| b | def | 
| c | ghi | 
| d | jkl | 
| a | mno | 
| e | pqr | 
| c | stu | 
| f | vwx | 

我想foo列刪除含重複所有行，以便表應該是這樣的：

| foo | bar | 
+-----+-----+ 
| b | def | 
| d | jkl | 
| e | pqr | 
| f | vwx | 

什麼是最有效如何做到這一點？

Answer 1

您可以使用LEFT JOIN僅返回唯一foo子查詢加入牌桌。你希望沒有在子查詢匹配的行會被刪除，例如

DELETE a 
FROM TableName a 
     LEFT JOIN 
     (
      SELECT foo 
      FROM TableName 
      GROUP BY Foo 
      HAVING COUNT(*) = 1 
     ) b ON a.Foo = b.Foo 
WHERE b.Foo IS NULL 

• SQLFiddle Demo

爲了更快的性能，在Foo列中添加索引。

ALTER TABLE tableName ADD INDEX(foo) 

Answer 2

使用EXISTS：

DELETE a 
    FROM TableName a 
WHERE EXISTS (SELECT NULL 
       FROM TableName b 
       WHERE b.foo = a.foo 
      GROUP BY b.foo 
       HAVING COUNT(*) > 1) 

使用IN：

DELETE a 
    FROM TableName a 
WHERE a.foo IN (SELECT b.foo 
        FROM TableName b 
       GROUP BY b.foo 
       HAVING COUNT(*) > 1)