如何通過一次查詢選擇父級和子級類別？

Question

我有兩張桌子第一張桌子擁有學校和分支機構的部門。 如果Dept和ChildDept值為0 ，則為部門。 如果Dept不同於0 ，則它是部門的分支。 ParentDept ， Dept和ChildDept值是不相關的deptID 。 據我了解，它們只是用於平整。

表：部門

    deptID     ParentDept     Dept     ChildDept Active Department
----------+--------------+--------+-------------+------+--------
       100|             1|       0|            0|     1|Education
       200|             1|       1|            0|     1|Primary School
       300|             1|       2|            0|     1|Primary Science
       315|             2|       0|            0|     1|Arts
       517|             2|       1|            0|     1|Painting
       518|             2|       2|            0|     1|Music
       555|             2|       3|            0|     0|Dance

第二張表保存學生信息

表：學生

    studID         deptID  
----------+--------------- 
         1|            300
         2|            200
         3|            517
         4|            200
         5|            300
         6|            517
         7|            518

我寫了一個SQL查詢，它獲取每個部門的學生人數：

SELECT d.Department,
       (SELECT COUNT(s.studID) 
        FROM Students AS s 
        WHERE d.deptID = s.deptID) AS studentCount,

       (SELECT Department 
        FROM Departments 
        WHERE deptID = ParentDept) AS ParentDeptName
FROM Departments AS d 
WHERE d.Active = 1

結果表如下：

Department      studentCount ParentDeptName
---------------+------------+--------------
Primary Science|           2|Education
Primary School |           2|Education
Painting       |           2|Education
Music          |           1|Education

如您所見，“繪畫”和“音樂”分支屬於“藝術”部門，但查詢結果顯示每一行的父部門的“教育”，這是錯誤的。 應該像

Department      studentCount ParentDeptName
---------------+------------+--------------
Primary Science|           2|Education
Primary School |           2|Education
Painting       |           2|Arts
Music          |           1|Arts

這個查詢或我怎么了？

Answer 1

試試這個：

SELECT d.Department,
   (SELECT COUNT(s.studID) 
    FROM Students AS s 
    WHERE d.deptID = s.deptID) AS studentCount,
   de.Departments AS ParentDeptName
FROM Departments AS d left join Departments AS de on d.ParentDept = de.deptID 
WHERE d.Active = 1

Answer 2

簽出我的小提琴，我不知道我要怎么數:(

SELECT (Select Department from Departments a where a.deptID = Departments.ParentDept) As ParentDEPTS,
(Select Department from Departments b where b.deptID = Students.deptID) As StudentDEPTS,  Students.studID AS STUIDS
 FROM Departments
 LEFT JOIN Students
ON Departments.deptID=Students.deptID 
Where 
    Departments.ParentDept is not null 
    AND 
    Students.deptID is not null
    AND 
    Departments.Active =1;

Answer 3

讓我們做一些測試數據：

DECLARE @Dept TABLE
(
    deptID INT,
    ParentDept INT,
    Dept INT,     
    ChildDept INT,
    Active INT, 
    Department VARCHAR(50)
)

INSERT INTO @Dept
VALUES
(100, 1, 0, 0, 1, 'Education'),
(200, 1, 1, 0, 1, 'Primary School'),
(300, 1, 2, 0, 1, 'Primary Science'),
(315, 2, 0, 0, 1, 'Arts'),
(517, 2, 1, 0, 1, 'Painting'),
(518, 2, 2, 0, 1, 'Music'),
(555, 2, 3, 0, 0, 'Dance');

DECLARE @Student TABLE
(
    studID INT,
    deptID INT
)

INSERT INTO @Student
VALUES
(1, 300),
(2, 200),
(3, 517),
(4, 200),
(5, 300),
(6, 517),
(7, 518);

現在，我們將與學生一起加入部門，然后將部門合並到父部門並將其分組以得到我們的結果：

SELECT 
    d.Department, 
    COUNT(*) RecCnt, 
    p.Department ParentDept 
FROM @Student s
INNER JOIN @Dept d
    ON s.deptID = d.deptID
INNER JOIN @Dept p
    ON p.ParentDept = d.ParentDept
    AND p.Dept = 0
GROUP BY d.Department, p.Department

這是輸出：

Department  RecCnt  ParentDept
Music           1   Arts
Painting        2   Arts
Primary School  2   Education
Primary Science 2   Education