選擇SQL困擾

Question

我有一個SQL查詢，如下所示：

select a.class_id, b.std_id, 
    (select count(*) from enrollment where class_id = a.class_id) as class_size, 
    d.admission_id 
    from class a, student b, enrollment c, admission d 
    where 
    c.class_id = a.class_id and 
    c.std_id = d.std_id and 
    c.std_id = b.std_id 
    order by a.class_id; 

結果是：

class_id std_id class_size 
comp100 8080 4 
comp100 8020 4 
comp100 8033 4 
comp100 8111 4 
comp305 8080 4 
comp305 8080 4 
comp305 8020 4 
comp305 8033 4 
comp555 8111 1 
comp672 8080 3 
comp672 8033 3 
comp672 8111 3 

但我只是想顯示的結果之一，如果每班人數超過1

是

我想這樣做：

class_id std_id class_size 
comp100 8080 4 
comp305 8080 4 
comp555 8111 1 
comp672 8080 3 

思考了一段時間之後，我應該先拿到專櫃，這樣

select enrollment.class_id, count(*) as class_size 
from class, enrollment 
where enrollment.class_id = class.class_id 
group by enrollment.class_id; 

則計數器被檢索如下：

class_id class_size 
comp100 4 
comp305 4 
comp555 1 
comp672 3 

然而

，我不知道如何使用一條SQL語句來實現結果。

我想問一下您的意見，如何修改SQL語句，這樣我就可以只顯示行之一，如果每班人數超過1

也就是說，如果班級規模4，我不想顯示4個結果，相反，我的結果就夠了。

非常感謝！

Answer 1

這可能不是最漂亮的查詢，但它應該得到你所問的：

select distinct class_id, std_id, class_size 
from (
    select a.class_id, b.std_id, 
     (select count(*) from enrollment where class_id = a.class_id) as class_size, 
     d.admission_id 
    from class a, student b, enrollment c, admission d 
    where 
     c.class_id = a.class_id and 
     c.std_id = d.std_id and 
     c.std_id = b.std_id) subquery1 
where 
    subquery1.class_size > 1 

union all 

select class_id, std_id, class_size 
from (
    select a.class_id, b.std_id, 
     (select count(*) from enrollment where class_id = a.class_id) as class_size, 
     d.admission_id 
    from class a, student b, enrollment c, admission d 
    where 
     c.class_id = a.class_id and 
     c.std_id = d.std_id and 
     c.std_id = b.std_id) subquery2 
where 
    subquery2.class_size = 1 

order by class_id