Question

我想从SentBy和SentTo的组合中选择最大ID 我的意思是我希望组（SentTo-SentBy）如1-3和3-1组合在一起..类似1-2和2-1也应该组合在一起

查询是什么？

用户：

**ID | Name** 

1 | A 

2 | B

3 | C

4 | D

聊天

ID | Msg | SentBy (Users.ID) | SentTo (Users.ID)

1 | abc | 1 | 2 

2 | def | 1 | 3

3 | ghi | 1 | 4

4 | jkl | 2 | 1

5 | mno| 2 | 1

6 | pqr | 3 | 1

7 | stu | 3 | 1

8 | vwx| 4 | 1

9 | yz | 4 | 1

10| def | 1 | 3

12| jkl | 2 | 1

必需

ID | Msg | SentBy (Users.ID) | SentTo (Users.ID)

3 | ghi | 1 | 4

10| def | 1 | 3

12| jkl | 2 | 1

Answer 1

使用LEAST()和GREATEST()

<强> Sql Fiddle Demo

SELECT Chat.*
FROM Chat
JOIN (
      SELECT MAX(`ID`) m_value
      FROM Chat
      GROUP BY LEAST(`SentBy`, `SentTo`) , GREATEST(`SentBy`, `SentTo`)
     ) C
  ON Chat.`ID` = C.m_value

<强>输出

| ID | Msg | SentBy | SentTo |
|----|-----|--------|--------|
|  9 |  yz |      4 |      1 |
| 10 | def |      1 |      3 |
| 12 | jkl |      2 |      1 |

Answer 2

我认为你正在寻找这个

select max(id) from chat as c1 
inner join chat as c2 on (c1.sent_by = c2.sent_to and c1.sent_to = c2_sent_by) 
group by c1.sent_by, c1.sent_to ;

我怎样才能从下表中选择一个不同的行

2 个答案: