子查询上的count函数的SQL查询问题

Question

我有两张桌子

BatchDetails

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

AdmissionBatch

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

我需要这些批次的BatchCode没有最大学生。我写了一个查询，但它无法正常工作

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

此查询无效，因为如果特定批次中没有学生，则批次将不会显示

我也试过了子查询，但没有帮助

请帮忙

Answer 1

使用左连接而不是内连接。

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

即使没有匹配项，您也需要左联接才能保留结果：

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