Moodle数据库的几个表之间的条件连接答案

【问题标题】：conditional join between several tables of moodle's databaseMoodle数据库的几个表之间的条件连接
【发布时间】：2019-08-10 02:43:49
【问题描述】：

我需要根据前一个表的输出在几个表之间创建一个连接查询。我正在尝试使用 case when 语句如下，但它显示错误！任何形式的帮助表示赞赏

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

【问题讨论】：

请在代码问题中给出minimal reproducible example--剪切&粘贴&运行代码；具有期望和实际输出的示例输入（包括逐字错误消息）；标签；明确的规范和解释。这包括您可以提供的最少代码，即您显示的代码可以通过您显示的代码扩展为不可以。（调试基础。） PS 你有一个语法错误。阅读语法和手册。显示组成子表达式是可以的。明确说明您的问题是关于 那个错误 并在稍后的新帖子中询问您的总体目标。 PS 显然这里有非最小代码/数据。
请use text, not images/links, for text--including tables & ERDs. 解释或引用其他文本。仅将图像用于无法表达为文本或增强文本的内容。无法搜索或剪切和粘贴图像。在图像中包含图例/键和说明。让您的帖子自成一体。

标签： mysql join conditional-statements moodle

【解决方案1】：

没有“条件连接”之类的东西。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

【讨论】：

【解决方案2】：

希望对你有所帮助-

SELECT completion.userid, completion. coursemoduleid, completion. timemodified, 
module.course, user. idnumber as student_id, m.name as module_name,(CASE WHEN (a.name != '') THEN a.name ELSE (CASE WHEN (ass.name != '') THEN ass.name ELSE q.name END) END) 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
left join mdl_assign as a on a.id = m.id
left join mdl_assignment as ass on ass.id = m.id
left join mdl_quiz as q on q.id = m.id
where m.name in ('assign','assignment','quiz')
LIMIT 0, 30

【讨论】：