如何在子查询中 select 多个值，但将每个值分配给匹配的父 select 值？

Question

抱歉，问题罗嗦。我有一个从 User table 中提取数据的查询，我想从 [ 中提取 completionstatecourse_modules_completion table 对于每个用户和课程中的每个 activity。我无法将来自各种 table 的多个值链接到每行的一个用户。

我能够从 User table 那里获得我想要的信息就好了。然后我加入多个 table，以便能够从 course_modules_completion table 中提取完成状态字段。我需要做的是为每个用户提取课程中每个 activity 对应的 completionstate 的值。

SELECT u.username AS 'ID'
,u.firstname AS 'Names'
,u.lastname AS 'Lastnames'

,(SELECT completionstate FROM prefix_course_modules_completion cmc
  INNER JOIN prefix_user u ON cmc.userid = u.id
  INNER JOIN prefix_course_modules cm ON cmc.coursemoduleid = cm.id
  INNER JOIN prefix_course c ON cm.course = c.id
  INNER JOIN prefix_resource r ON c.id = r.course
  WHERE (r.id = 6) AND (c.id = 5)
) AS 'Activity 1'

FROM prefix_user u LEFT JOIN prefix_grade_grades g ON u.id = g.userid
INNER JOIN prefix_course_modules_completion cmc ON cmc.userid = u.id
INNER JOIN prefix_course_modules cm ON cmc.coursemoduleid = cm.id
INNER JOIN prefix_course c ON cm.course = c.id
INNER JOIN prefix_resource r ON c.id = r.course

WHERE (u.id > 7) AND (u.firstaccess > 0) AND (c.id = 5)                                     

GROUP BY u.username
ORDER BY u.firstname ASC

我期望的是这样的：

ID  - Name - Last name - Email   -  Activity 1 - Activity 2 - Test
382 - John - Johnson   - email@e.com  - Seen  -  Not seen  -  Failed
17  - Mark - Markson   - email2@e.com - Seen  -    Seen    -  Passed

相反，我得到的是同一用户的多行，具有所有用户想要的值或相同的值，就好像它没有对用户进行任何区分一样。正如您在上面的代码中看到的那样，我尝试使用仅针对 completionstate 值的子查询，但它返回多行，因此出现错误。

此外，我想将 completionstate 列中的值显示为文本。该列中有四个值：0、1、2、3，我想做的是将每个值转换为 "Not seen"、"Seen"、"Completed and passed" 和 "Completed and failed"分别

因为我是 SQL 的初学者，所以我无法确定我的问题是由于错误的逻辑、错误的编码还是两者兼而有之。

编辑：稍微删减了代码，使这个问题更简单。

Answer 1

您尝试做的是一种枢轴，因此您可以在没有相关子查询的情况下完成。

SELECT u.username AS 'ID'
        ,u.firstname AS 'Names'
        ,u.lastname AS 'Lastnames'
        ,u.email AS 'Email'
        ,c.fullname AS 'Course'
        ,(LEFT(g.finalgrade,2)) AS 'Final grade'
        ,ELT(MAX(IF(r.id = 6, completion_state, NULL))+1,
            'Not seen', 'Seen', 'Completed and passed', 'Completed and failed') AS `Activity 1`
        ,ELT(MAX(IF(r.id = 7, completion_state, NULL))+1,
            'Not seen', 'Seen', 'Completed and passed', 'Completed and failed') AS `Activity 2`
        ,ELT(MAX(IF(r.id = 8, completion_state, NULL))+1,
            'Not seen', 'Seen', 'Completed and passed', 'Completed and failed') AS `Activity 3`
FROM prefix_user u LEFT JOIN prefix_grade_grades g ON u.id = g.userid
INNER JOIN prefix_course_modules_completion cmc ON cmc.userid = u.id
INNER JOIN prefix_course_modules cm ON cmc.coursemoduleid = cm.id
INNER JOIN prefix_course c ON cm.course = c.id
INNER JOIN prefix_resource r ON c.id = r.course

WHERE (u.id > 7) AND (u.firstaccess > 0) AND (c.id = 5)                                     

GROUP BY u.id
ORDER BY u.firstname ASC

你还应该按 u.id 而不是 u.username 分组。这将允许您 select 在功能上依赖于此主键的其他列，而无需使用聚合函数。

如何在子查询中 select 多个值，但将每个值分配给匹配的父 select 值？

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