Question

这是我的查询：

SELECT id, lastname, firstname,

( SELECT COUNT( courses.id )
FROM courses
LEFT JOIN members_courses
ON courses.id = members_courses.course_id
WHERE members_courses.member_id = members.id ) as attended_courses

FROM members

LIMIT 10;

它工作正常，但现在我只需要选择在attend_courses = 0;

的行

我该怎么做？

三江源！

Answer 1

尝试类似这样的想法是让子查询作为一个表工作，这样你可能需要将member.id = member_courses.memeber_id置于子查询之外，尝试两种方式。：

SELECT id, lastname, firstname,cnt 
from  ( SELECT COUNT( courses.id ) as cnt FROM courses 
        LEFT JOIN members_courses ON courses.id = members_courses.course_id
        WHERE members_courses.member_id = members.id ) attended_courses, 
members 
where cnt=0 ;

Answer 2

HAVING关键字用于后处理，类似于WHERE，用于预处理。

SELECT id, lastname, firstname,

( SELECT COUNT( courses.id )
FROM courses
LEFT JOIN members_courses
ON courses.id = members_courses.course_id
WHERE members_courses.member_id = members.id ) as attended_courses

FROM members
HAVING attended_courses=0
LIMIT 10;

Answer 3

您可以使用连接重写您的查询

SELECT m.id, m.firstname, m.lastname , COUNT( courses.id ) attended_courses
FROM members m
LEFT JOIN members_courses mc ON mc.member_id = m.id
LEFT JOIN courses c ON c.id = mc.course_id
GROUP BY m.id, m.firstname, m.lastname
HAVING  attended_courses = 0

任何人都可以帮助我使用Mysql子查询？

3 个答案: