此子查询是否可以写入连接并仍然获得相同的结果集/行数？

Question

如何将以下内容写入连接并获取相同数量的行？

SELECT 
    s.subjectid, 
    s.subjectname, 
    (SELECT 
    COUNT(*) 
    FROM dbo.Classes AS c 
    WHERE c.SubjectID = s.SubjectID 
    AND c.MondaySchedule = 1) 
    AS numofclasses 
FROM dbo.subjects AS s 
ORDER BY numofclasses DESC 

我试图把它写像下面，但得到了不同的答案：

SELECT 
    s.subjectid, 
    COUNT(ClassID) AS numberofclasses 
FROM dbo.subjects AS s 
LEFT JOIN dbo.classes AS c 
    ON s.SubjectID = c.SubjectID 
WHERE c.MondaySchedule = 1 
GROUP BY s.Subjectid 
ORDER BY numberofclasses DESC 

Answer 1

移动where条件的on条件。据转换外部联接于内连接：

select s.subjectid, count(ClassID) as numberofclasses 
from dbo.subjects s left join 
    dbo.classes c 
    on s.SubjectID = c.SubjectID and c.MondaySchedule = 1 
group by s.Subjectid 
order by numberofclasses desc ; 

这并假定subjects(subjectid)是唯一的（或主键）。否则，这两者可能会返回不同的结果。