被MySQL子查询难住了

假设我有一张这样的桌子：

表用户注册

username     role     class
John Doe     student  Algebra 1
Jane Duh     student  Algebra 1
Jeff Doh     student  Geometry
Jack Sprat   student  Algebra 1
Willie Winky student  Geometry
Donald Duck  teacher  Algebra 1
Mickey Mouse teacher  Geometry

我想要这些结果：

# of students   Class name  Teacher name
3                Algebra 1   Donald Duck
2                Geometry    Mickey Mouse

我将如何编写查询

(SELECT COUNT ue.username
FROM user_enrollments AS ue
WHERE ue.role="student") AS "# of Students",
ue.class AS "Class name",
(SELECT ue.username
FROM user_enrollments AS ue
WHERE ue.role="teacher") AS "Teacher name"

当然，上面的查询不起作用，但我不确定该采取什么方法

上表大大简化了。在我的实际示例中，该表是8个联接的结果。这更接近我真正的疑问：

(SELECT COUNT u.username
WHERE ra.roleid='5') AS "Number of Students",
c.fullname AS "Course name",
(SELECT u.username
WHERE ra.roleid='3') AS "Teacher name"

FROM prefix_user u
JOIN prefix_user_enrolments ue on ue.userid=u.id
JOIN prefix_enrol e ON e.id=ue.enrolid
JOIN prefix_course c on c.id = e.courseid
JOIN prefix_context AS ctx ON ctx.instanceid = c.id
JOIN prefix_role_assignments AS ra ON ra.contextid = ctx.id
JOIN prefix_role AS r ON r.id = e.roleid
JOIN prefix_course_categories AS cc ON cc.id=c.category

WHERE ra.userid=u.id

GROUP BY c.id

我知道子查询在编写时无法正常工作，但我不知道如何正确编写它们，我已经包含了WHERE子句，以便您了解我需要的要点

我也知道这似乎是在连接不必要的表，但其中一些表是连接两个不共享公共列的表所必需的

谢谢你的帮助！ 金伯

• 现场观看它的工作

试试这个

select class as [Class Name], username as [Teacher Name],
(
    select count(username)
    from table_user_enrollments
    where role = 'student'
    and class = e.class
) as [No of Student]
from table_user_enrollments e
where role = 'teacher' 

---

这将为您提供您想要的：

select 
    count(t1.username) as "# of students",
    t1.class as "class name",
    t2.username as "Teacher name"
from
    table_user_enrollments t1
       left join
    table_user_enrollments t2 
on
    t1.class = t2.class
where t1.role = 'student' 
and t2.role = 'teacher'
group by t1.class

---

如果唐老鸭教我代数，我会非常喜欢。有时，一个更简单的解决方案会从更复杂的问题中显现出来：你的实际表的结构是什么？如果继续这样做，还可以避免形成一个工作查询，但却无法在真正的表上工作。这太棒了！我不知道表可以通过特定的值连接起来，比如在teachers.role='teacher'和student.role='student'上的内部连接。我只见过像老师一样的学生。如果您不介意解释一下，您使用的join语句会发生什么？我能看出它是有效的，但我不明白它为什么有效。我特别感谢包含到sqlfiddle的链接。谢谢实际上，你可以把

teachers.role='teacher'和students.role='student'

也放到

where

子句中。我只是习惯于把逻辑放在它应该放的地方。有时，也需要将它们置于联接条件中，例如，当您

左联接时。在
where
子句中使用上述过滤器实际上会使
左连接成为
内部连接。这就是为什么我习惯了。下面是关于连接的一个很好的解释：
select 
    count(t1.username) as "# of students",
    t1.class as "class name",
    t2.username as "Teacher name"
from
    table_user_enrollments t1
       left join
    table_user_enrollments t2 
on
    t1.class = t2.class
where t1.role = 'student' 
and t2.role = 'teacher'
group by t1.class