Question

假设我有一个这样的表：

table_user_enrollments

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，但我不确定采取什么方法。

上表大大简化了。在我的现实生活中，该表是8个连接的结果。这更接近我的真实问题：

(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不起作用，但我不知道如何正确编写它们，并且我已经包含了WHERE子句，因此您可以获得我需要的要点。

我也知道可能看起来我正在加入不必要的表，但其中一些是必要的，可以作为连接两个不共享公共列的表的桥梁。

感谢您的帮助！金伯

Answer 1

select
  teachers.username,
  teachers.class,
  count(*) as nr_of_students
from
  user_enrollments teachers
inner join user_enrollments students on 
    teachers.role = 'teacher' and
    students.role = 'student' and
    teachers.class = students.class
group by 
  teachers.username,
  teachers.class

看到它在 sqlfiddle

Answer 2

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'

Answer 3

这将为您提供所需的信息：

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

被MySQL子查询困扰

3 个答案: