MySQL删除重复的反向值

Question

我有MySQL MyISAM表:

表朋友(id,friend_id):

1,5

5,1

2,6

6,2

3,7

如何删除反向记录？如果记录值"1,5"存在值为"5,1"的记录,则需要删除"5,1".

Thanx寻求帮助!

Answer 1

DELETE F1
FROM friends F1, friends F2
WHERE F1.friend_id = F2.id
  AND F2.friend_id = F1.id
  AND F1.id > F1.friend_id

编辑

一个更好的语法将是:

DELETE F1
FROM friends F1
  JOIN friends F2 ON F1.friend_id = F2.id AND F2.friend_id = F1.id
WHERE F1.id > F1.friend_id

但执行时间是一样的.

同样,我已经创建了这个小脚本,用于快速和肮脏的基准测试.

结果:

没有索引:

Dalen: 600 => 400 rows. Time: 0.0274
Mark: 600 => 400 rows. Time: 0.4323
Frosty: 600 => 400 rows. Time: 0.4081
Nick: 600 => 400 rows. Time: 0.3201

列上的单独索引id和friend_id:

Dalen: 600 => 400 rows. Time: 0.0201
Mark: 600 => 400 rows. Time: 0.0095
Frosty: 600 => 400 rows. Time: 0.0059
Nick: 600 => 400 rows. Time: 0.3257

独特索引(id, friend_id):

Dalen: 600 => 400 rows. Time: 0.0168
Mark: 600 => 400 rows. Time: 0.0057
Frosty: 600 => 400 rows. Time: 0.0041
Nick: 600 => 400 rows. Time: 0.3209

结论:

• Dalen:列未编入索引时速度最快
• Frosty:列索引时速度最快(Mark关闭,在"索引情况"中更快,而不考虑tmp表创建时间.但是,当字段被索引时,tmp表创建所需的额外时间会增加)