Question

我有很多表代表可以“参与”的小组（比如俱乐部，社团和班级），以及代表所有“参与”（参与）的表格。因此，我可以找出用户参与的课程如下：

SELECT * FROM Participations WHERE user_id = 1 and Participations.group_type = "Class";

我还可以通过这样的方式找出哪个用户在俱乐部中的ID为12：

SELECT * FROM Users INNER JOIN Participations ON (Users.id = Participations.user_id) WHERE Participations.group_type = "Club" and Participations.group_id = 12;

我想找到所有在Club 12，Club 20或Class 10中的用户，而不返回任何未参与任何这些组的用户。

SELECT * FROM Users INNER JOIN Participations ON (Users.id = Participations.user_id) WHERE (Participations.group_type = "Club" AND Participations.group_id IS IN (12,20)) OR (Participations.group_type = "Class" AND Participations.group_id IS 10)

此外，我想显示他们参与的群组的名称，同样不包括任何未参与任何这些群组的用户（如下所示）：

SELECT Users.*, Classes.name, Clubs.name, FROM Users, Classes, Clubs, Participations (Users.id = Participations.user_id) WHERE Users.id = Participations.user_id AND ((Participations.group_type = "Club" AND Participations.group_id IS IN (12,20)) OR (Participations.group_type = "Class" AND Participations.group_id IS 10))

但我更喜欢使用显式连接。它可行吗？

Answer 1

您只需向join子句添加所需条件：

SELECT * FROM Users 
 INNER JOIN Participations 
    ON (Users.id = Participations.user_id) 
   AND ( (Participations.group_type = "Club" AND Participations.group_id IS IN (12,20)) 
      OR (Participations.group_type = "Class" AND Participations.group_id IS 10) )

如何在不拉回空白记录的情况下加入多个多态表？

1 个答案: