多个查询如何加入它

问题描述

我有2张桌子，但我想加入他们。怎么做



选择classSchedule。[classScheduleId]来自classSchedule



classScheduleId

------------

1000

1001

1002



选择classSchedule.classScheduleId，count（attendance.studentId）as attend where attendance.classScheduleId = classSchedule.classScheduleId group by（classSchedule.classScheduleId）



classScheduleId |参加

--------------------------

1001 | 2





但我期待的是



classScheduleId |参加

--------------------------

1000 | 0

1001 | 2

1002 | 0



我的尝试：



在这种情况下，我不知道如何做到这一点，我只是想找到一些例子来解决它，但直到现在我也不知道我要搜索的关键字

i have 2 table but I want join them together. how to do it

select classSchedule.[classScheduleId ] from classSchedule

classScheduleId
------------
1000
1001
1002

select classSchedule.classScheduleId ,count(attendance.studentId) as attend where attendance.classScheduleId=classSchedule.classScheduleId group by(classSchedule.classScheduleId )

classScheduleId | attend
--------------------------
1001 | 2


but what i expect is

classScheduleId | attend
--------------------------
1000 | 0
1001 | 2
1002 | 0

What I have tried:

In this case i dont know how to do so that i just try to find some example to solve it but until now i also dont know the keyword that i want to search

推荐答案

SELECT s.classScheduleID, COUNT(a.classScheduleId) FROM classSchedule s
LEFT JOIN Attendance a ON a.classScheduleId = s.classScheduleID
GROUP BY s.classScheduleId

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