MySQL内连接获取连接表的最后一项

Question

有两个表：

表名：ticket

+-------+---------+---------+-------------------+
|  id   |  title       |  closed    | param     |
+-------+--------------+------------+-----------+
| 1     | test1        | 0          | 1         |
| 2     | test2        | 1          | 1         |
| 3     | test3        | 0          | 1         |

表名：ticket_event

+-------+---------+---------+-----------------+
|  id   |  ticket_id   |  event   | date      |
+-------+--------------+----------+-----------+
| 1     | 1            | OPENED   |           |
| 2     | 1            | CLOSED   |           |
| 3     | 2            | OPENED   |           |
+-------+--------------+----------+-----------+

我需要获取每张票的最后一次 ticket_event 记录。结果应该是：

test1 CLOSED
test2 OPENED
test3 NULL

我尝试了这段代码，但我得到了 ticket_event 的所有记录：

SELECT t1.title as t1Title, t2.event as t2Event FROM ticket t1 LEFT JOIN ticket_event t2 ON t1.id = t2.ticket_id

Answer 1

您可以将ticket 加入到返回最后一个事件的查询中：：

SELECT t.title, e.event
FROM ticket t LEFT JOIN (
  SELECT e.* from ticket_event e
  WHERE NOT EXISTS (
    SELECT 1 FROM ticket_event 
    WHERE ticket_id = e.ticket_id AND id > e.id
  )
) e 
ON t.id = e.ticket_id

请参阅demo。
结果：

| title | event  |
| ----- | ------ |
| test1 | CLOSED |
| test2 | OPENED |
| test3 |        |

Answer 2

相关子查询可以解决问题：

select 
    t.title,
    (
        select te.event 
        from ticket_event te 
        where te.ticket_id = t.id 
        order by te.id desc 
        limit 1
    ) last_event
from ticket t

目前还不清楚您想使用ticket_event 中的哪一列进行排序。使用date 是有意义的，但是您的表格中该列是空的，所以我选择了id。您可以根据自己的实际需求进行更改。