有關by子句

Question

在組計數記錄的問題，我必須有StudentId(int),Subject(varchar(50)),Marks(int),IsPass(int),ExamDate(Datetime)

表可以有針對不同日期特定學生的科目多條記錄一個數據庫表。

我寫了下面的查詢：

select StudentId, Count(IsPass) 
from ExamEntry 
where IsPass =1 group by StudentId 

但不想當條件查詢：

有可能是這樣的：

Select StudentId, case when IsPass = 1 then count(IsPass) end 
from ExamEntry 
group by studentId 

，但它顯示更多的一個記錄對於特殊學生Id

我該如何實現我的目標？

Answer 1

如果您不想使用where子句，那麼Having子句是其最簡單的替代方法。

試試看。

select StudentId, count(IsPass)AS PassedCount 
from ExamEntry 
group by StudentId,IsPass 
having IsPass= 1 

Answer 2

SELECT StudentId, 
     COUNT(CASE WHEN IsPass = 1 THEN 'X' END) AS NumberOfPasses 
FROM ExamEntry 
GROUP BY StudentId 

COUNT只能算作NOT NULL值和CASE有一個隱ELSE NULL

Answer 3

也許你需要這樣的事？

SELECT StudentId, SUM(CASE WHEN IsPass = 1 THEN 1 ELSE 0 END) AS Number_Of_Passed_Exams 
FROM ExamEntry 
GROUP BY StudentId 

Answer 4

答案就是這麼簡單沒有人看到它:-)

SELECT StudentId, SUM(IsPass) 
FROM ExamEntry GROUP BY StudentId