Question

我有这个问题的例子： http://sqlfiddle.com/#!9/0cc41/1/0

描述：有三个表（事件，人员和用户）。表人员连接事件和用户。 persons.type_id是events.id，persons.user_id是user.id.我创建了两个事件（id 1和2）。每个活动都有一个人参加。

我的Sql：

SELECT events.*, 
coalesce(part_person.Accept_Participants_LJ, 0) AS Accept_Participants
FROM events 
LEFT JOIN (
    SELECT GROUP_CONCAT(CONCAT(part_user.forname, ' ', part_user.surname) SEPARATOR ', ') AS Accept_Participants_LJ,
    part_person.type_id
    FROM persons AS part_person
    LEFT JOIN user AS part_user ON part_user.id = part_person.user_id 
    WHERE part_person.type = 'event_participant'
) part_person ON events.id = part_person.type_id
GROUP BY events.id

我的期望是：

------------------------
|id|Accept_Participants|
------------------------
|1 | Carl Habicht      |
------------------------
|2 | Peter Zwegert     |
------------------------

如您所见，结果是：

----------------------------------
|id| Accept_Participants         | 
----------------------------------
|1 | Carl Habicht,  Peter Zwegert|
----------------------------------
|2 | 0                           |
----------------------------------

看来，他忽略了左连接的开启条件。

但是，我的错误在哪里？

Answer 1

Group_concat应该（几乎）总是有一个group by和group by而没有任何聚合是没用的也许这就是你需要的

SELECT events.*, 
coalesce(part_person.Accept_Participants_LJ, 0) AS Accept_Participants
FROM events 
LEFT JOIN (
    SELECT GROUP_CONCAT(CONCAT(part_user.forname, ' ', part_user.surname) SEPARATOR ', ') AS Accept_Participants_LJ,
    part_person.type_id
    FROM persons AS part_person
    LEFT JOIN user AS part_user ON part_user.id = part_person.user_id 
    WHERE part_person.type = 'event_participant'
    group by part_person.type_id 
) part_person ON events.id = part_person.type_id
order  BY events.id

MYSQL Join Left Condition不起作用

1 个答案: