SQL查詢問題的計數功能上的子查詢

Question

我有兩個表

BatchDetails

BatchCode MaxStd 
------------------ 
B001   12 
B002   14 
B003   10 

AdmissionBatch

Batch Rollno 
--------------- 
B001 1 
B001 2 
B002 3 
B003 4 
B003 5 

我需要這些批次的BatchCode都沒有最多的學生。我寫了一個查詢，但它不工作

select 
    batchCode, MaxStd, count(rollno) 
from 
    BatchDetails as a 
join 
    AdmissionBatch as b on a.batchcode = b.batch 
group by 
    batchcode, maxStd 
having 
    count(rollno) < maxSTD 

此查詢不是因爲工作，如果沒有學生在某一批次，則批次將不會出現

我試圖子查詢很好，但沒有幫助

請幫忙

Answer 1

使用左連接而不是內連接。

select batchCode,MaxStd, count(rollno) from BatchDetails as a 
LEFT JOIN AdmissionBatch as b on a.batchcode=b.batch 
group by batchcode,maxStd 
having count(rollno)< maxSTD 

Answer 2

你需要一個左連接，以保持結果，即使沒有匹配：

select batchCode,MaxStd, count(rollno) 
from BatchDetails as a 
left outer join AdmissionBatch as b 
on a.batchcode=b.batch 
group by batchcode,maxStd 
having count(rollno)< maxSTD