來自子選擇查詢的意外答案

Question

當我執行此查詢時，所有出發部門都顯示在depart_decode列中，那里的數字也= Nil

select D.depart_decode 
(select count (Staff_NO) from AA.table where Depart_code=D.Depart_code and Depart_code=151 and Staff_Sub_NO=5) Manager,
(select count (Staff_NO) from AA.table where Depart_code=D.Depart_code and Depart_code=151 and Staff_Sub_NO=4) HOD,
(select count (Staff_NO) from AA.table where Depart_code=D.Depart_code and Depart_code=151 and Staff_Sub_NO=3) Head,
(select count (Staff_NO) from AA.table where Depart_code=D.Depart_code and Depart_code=151 and Staff_Sub_NO in(1,2)) staff
 from AA.Departments D

Answer 1

您查詢的每個子選擇中都包含以下where子句：

where Depart_code=D.Depart_code and Depart_code=151

僅當表Departments中存在Depart_code 151並且僅對該行存在時，此結果才為true。

刪除and Depart_code=151 ，您應該得到結果。

Answer 2

在select clause使用correlated subqueries通常是性能問題的根源。 我建議使用子查詢來准備所需的計數，然后將其重新加入到Department表中。

SELECT
      D.depart_decode, c.Manager, c.Hod, c.Head, c.staff
FROM AA.Departments D
INNER JOIN (
            SELECT
                  Depart_code
                , COUNT(CASE WHEN Staff_Sub_NO = 5 THEN Staff_NO END) Manager
                , COUNT(CASE WHEN Staff_Sub_NO = 4 THEN Staff_NO END) HOD
                , COUNT(CASE WHEN Staff_Sub_NO = 3 THEN Staff_NO END) Head
                , COUNT(CASE WHEN Staff_Sub_NO < 3 THEN Staff_NO END) staff
            FROM AA.table
            GROUP BY
                  Depart_code
            ) C ON D.Depart_code = C.Depart_code
;