此子查詢是否可以寫入連接並仍然獲得相同的結果集/行數？

Question

如何將以下內容寫入連接並獲取相同數量的行？

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 

我試圖把它寫像下面，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

移動where條件的on條件。據轉換外部聯接於內連接：

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 ; 

這並假定subjects(subjectid)是唯一的（或主鍵）。否則，這兩者可能會返回不同的結果。