mysql - 我可以在 MySQL 中将条件放入 INNER JOIN 中吗？

Question

我有两种类型的用户，我将它们保存在两个不同的表中，我需要根据类型选择该用户的数据，这是我的查询:

SELECT user_data_bis.name, user_data_bis.surname, 
       user_data_bis.avatarimage, user.externalid, user.type // User's Data
FROM user_wall_likes 
INNER JOIN user
    ON user.internalid = user_wall_likes.owner
INNER JOIN  (CASE user.type
                  WHEN user.type=0
                  THEN
                      (
                      SELECT name, owner, surname, avatarimage
                      FROM user_data
                      ) user_data_bis
                  WHEN user.type=1
                  THEN
                      (
                      SELECT name, "" AS surname, owner, icon AS avatarimage
                      FROM university_data
                      WHERE isdelete=0
                      ) user_data_bis
             END)
             ON user_data_bis.owner = user.internalid 
             WHERE user_wall_likes.wallid="619fd58836c9a826b9eafb4b4e81da24" 
                   AND user_wall_likes.isdeleted=0
             LIMIT 5

注意:当它说user_wall_likes.wallid="619fd58836c9a826b9eafb4b4e81da24时，这个varchar是我的数据库中一列的示例

但是当我执行这个查询时，它显示:

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'CASE user.type WHEN user.type=0' at line 6

在这种情况下，是否有某种方法可以根据变量user.type从表中提取数据？

Answer 1

您可以使用两个left join和coalesce()来完成此操作:

SELECT coalesce(ud.name, unv.name) as name,
       coalesce(ud.surname, unv.surname) as surname,
       coalesce(ud.avatarimage, unv.avatarimage) as avatarimage,
       u.externalid, u.type    -- User's Data
FROM user_wall_likes uld INNER JOIN
     user u
     ON u.internalid = uwl.owner LEFT JOIN
     user_data ud
     ON u.type = 0 AND ud.owner = u.internalid LEFT JOIN
     university_data unv
     ON u.type = 1 AND unv.owner = u.internalid
WHERE uwl.wallid = '619fd58836c9a826b9eafb4b4e81da24' AND uwl.isdeleted = 0
LIMIT 5;

请注意，您通常会有一个 ORDER BY 和 LIMIT。