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Question

我有一個員工表包含employeeID，name和ManagerID - 3列，我想寫一個查詢，顯示name，managerName和level，老闆沒有managerID，其managerID is null。
見下表結構

CREATE TABLE #employee (
    EmployeeID int, 
    [Name] nvarchar(50), 
    ManagerID int 
) 

INSERT INTO #employee VALUES 
(1,'Tom',2), 
(2,'Josh',NULL), 
(3,'Mike',2), 
(4,'John',3), 
(5,'Pam',1), 
(6,'Mary',3), 
(7,'James',1), 
(8,'Sam',5), 
(9,'Simon',1) 

我想要得到的效果：

我如何寫SQL查詢？

Answer 1

在SQL服務器你可以用遞歸CTE這樣做：

;WITH rec AS (
SELECT EmployeeID, 
     [Name], 
     ManagerID, 
     1 as [Level] 
FROM employee 
WHERE ManagerID IS NULL 
UNION ALL 
SELECT e.EmployeeID, 
     e.[Name], 
     e.ManagerID, 
     r.[Level] + 1 
FROM employee e 
INNER JOIN rec r 
    ON e.ManagerID = r.EmployeeID 
) 

SELECT r.[Name] as Employee, 
     COALESCE(e.[Name],'Super Boss') as Manager, 
     r.[Level] 
FROM rec r 
LEFT JOIN employee e 
    ON e.EmployeeID = r.ManagerID 

輸出：

Employee Manager  Level 
Josh  Super Boss 1 
Tom   Josh  2 
Mike  Josh  2 
John  Mike  3 
Mary  Mike  3 
Pam   Tom   3 
James  Tom   3 
Simon  Tom   3 
Sam   Pam   4