关于SQL Row过滤器

Question

我在sql中有这种类型的数据库表

id | state | msg | userid | time |
1  |unread | hi  |    1   | time |
2  |unread |hello|    1   | time | 
3  |unread | hi  |    2   | time |
4  | read  | hi  |    2   | time |

我已经开始打印行

Where state=unread

现在输出

= userid 1 send you 'hi'
= userid 1 send you 'hello'
= userid 2 send you 'hi'

但我想以这种方式输出

= userid 1 send you 'hello'
= userid 2 send you 'hi'

删除相同的用户标识行并仅选择最后一行（使用带sqli的php）

Answer 1

试试这个：

SELECT a.id, a.state, a.msg, a.userid, a.time 
FROM tableA a 
INNER JOIN (SELECT a.userid, MAX(a.id) id 
            FROM tableA a 
            WHERE a.state = 'unread'
            GROUP BY a.userid
           ) AS b ON a.userid = b.userid AND a.id = b.id;