MySQL - 加入同一张表两次

Question

我有一个名为“预订”的表

> id     status 
  1      P 
  2      P

还有一个叫做'call'

id    calldate    type   booking
1     01/01/2012  DEL    1
2     01/02/2012  COL    1
3     01/03/2012  DEL    2
4     31/12/2019  COL    999

我想一次列出“预订”中的每条记录，将来自“调用”的相关记录显示为另一列，如下所示:

bookingId    deliverydate  collectiondate
1            01/01/2012    01/02/2012
2            01/03/2012    null

我试过:

select `b`.`bookingid` AS `bookingid`,
       `del`.`calldate` AS `Delivery`,
       `col`.`calldate` AS `Collection`
from `booking` `b`
left join `call` `del` on `b`.`bookingid` = `del`.`booking`
left join `call` `col` on `b`.`bookingid` = `col`.`booking`
where ((`del`.`type` = 'DEL') OR (`col`.`type` = 'COL') and (`b`.`status` = 'P'));

但我得到 bookingid 1 列出 3 次。有人可以修复我的连接吗？

Answer 1

我认为您想将您的类型移动到连接条件中:

select `b`.`bookingid` AS `bookingid`,
   `del`.`calldate` AS `Delivery`,
   `col`.`calldate` AS `Collection`
from `booking` `b`
left join `call` `del` on `b`.`bookingid` = `del`.`booking` AND `del`.`type` = 'DEL'
left join `call` `col` on `b`.`bookingid` = `col`.`booking` AND `col`.`type` = 'COL'
where `b`.`status` = 'P';