mysql - 穆德尔数据库的几个表之间的条件连接

我需要基于一些先前表的输出在几个表之间创建一个联接查询。我正在尝试使用如下情况时的语句，但显示错误！任何帮助都值得赞赏

SELECT completion.userid, completion. coursemoduleid, completion. timemodified,
module.course, user. idnumber as student_id, m.name as module_name, activity.name as activity_name
FROM `mdl_course_modules_completion` as completion

join mdl_course_modules as module
on completion. coursemoduleid = module.id

join mdl_user as user on user.id = completion.userid

join mdl_modules as m on completion. coursemoduleid = m.id

join
CASE WHEN module_name = 'assign'THEN 'mdl_assign'
     WHEN module_name = 'assignment'  THEN 'mdl_assignment'
     ELSE "quiz"
END AS activity

on activity.id = m.id LIMIT 0, 30

最佳答案

没有“条件连接”这样的东西。相反，您可以使用两个LEFT JOIN和COALESCE（）：

SELECT completion.userid,
  ...
  COALESCE(activity1.name, activity2.name) as activity_name -- SELECT first non NULL value
FROM `mdl_course_modules_completion` as completion
...
LEFT JOIN mdl_assign activity1
  ON  activity1.id = m.id    -- JOIN condition
  AND module_name = 'assign' -- CASE condition
LEFT JOIN mdl_assignment activity2
  ON  activity2.id = m.id        -- JOIN condition
  AND module_name = 'assignment' -- CASE condition