SQL聚合函數的子查詢

Question

我想要做的就是數由子查詢返回的行數，主要是：

select pp.prop_id, COUNT((select employee_id from employee e where e.ao1_hours > 0)) 
    from proposal_piece pp 

    group by pp.prop_id 
    order by pp.prop_id 

這是我的錯誤信息：

Cannot perform an aggregate function on an expression containing an aggregate or a subquery. 

爲什麼這不行？如果select只是返回一堆帶有過濾條件的employee_id's，爲什麼我不能計算正在返回的行數或employee_id's？

我在計算有ao1_hours > 0的不同僱員的人數。按prop分組。

這是關於我的數據庫的一些結構信息，作爲查詢的一部分。

from proposal_piece pp 
    INNER JOIN employee e 
    on pp.employee_id = e.employee_id 

謝謝！

Answer 1

試試這個

select pp.prop_id, 
     (select COUNT(employee_id) 
     from employee e 
     where e.ao1_hours > 0 and e.employee_id = pp.employee_id) as nb_employees 
from proposal_piece pp  
order by pp.prop_id 

或本

select pp.prop_id, count(e.employee_id) as nb_employees 
from proposal_piece pp inner join employee e 
    on pp.employee_id = e.employee_id 
where e.ao1_hours > 0 
group by pp.prop_id 
order by pp.prop_id