SQL递归udf始终返回null

Question

Create Table Employees
(
    Employee varchar(10),
    Manager varchar(10)
);

Insert into Employees
values
('Charlie',null),
('Peter','James'),
('Elai',null),
('Graham','Emanuel'),
('Amanda','Charlie'),
('Sen','Graham'),
('Emanuel',null),
('James','Amanda'),
('Elai',null),
('Victor','Elai');

Above "Employees" table contains employee and employee's manager name. When trying to retrieve comma separated hierarchy of a employee using below function, the result is always null.

for example :
employee 'Victor' , hierarchy/result should be "Victor, Elai" .

Could anyone point out what am I doing wrong in below UDF.

Create Function EmployeeHierarchy(@employeeName varchar(20))
Returns varchar(100)
AS 
Begin
    Declare @commaSeparatedHierarchy varchar(100);
    Declare @manager varchar(20);

    if(@employeeName is not null)
    Begin       
        Select @manager=Manager from Employees where Employee=@employeeName;
        Set @commaSeparatedHierarchy=dbo.EmployeeHierarchy(@manager)+','+@manager;      
    End

    return @commaSeparatedHierarchy;
End;

Answer 1

First & foremost, you DO NOT want to create this as a scalar function. Their performance is horrible any udf you create, should be created as an inline table valued function. The following should do what you're looking for...

-- the test data...
USE tempdb;
GO 
IF OBJECT_ID('tempdb.dbo.Employee', 'U') IS NOT NULL 
DROP TABLE dbo.Employee;

CREATE TABLE dbo.Employee ( 
    Employee varchar(10),
    Manager varchar(10)
    );
INSERT dbo.Employee (Employee, Manager) VALUES
    ('Charlie',null),
    ('Peter','James'),
    ('Elai',null),
    ('Graham','Emanuel'),
    ('Amanda','Charlie'),
    ('Sen','Graham'),
    ('Emanuel',null),
    ('James','Amanda'),
    ('Elai',null),
    ('Victor','Elai');

SELECT * FROM dbo.Employee e;

iTVF code...

CREATE FUNCTION dbo.EmployeeHierarchy
(
    @employeeName varchar(20)
)
RETURNS TABLE WITH SCHEMABINDING AS
RETURN
    WITH
        cte_Recur AS (
            SELECT 
                CSH = CAST(CONCAT(e.Employee, ', ' + e.Manager) AS VARCHAR(1000)),
                e.Manager,
                NodeLevel = 1
            FROM
                dbo.Employee e
            WHERE 
                e.Employee = @employeeName
            UNION ALL 
            SELECT 
                CSH = CAST(CONCAT(r.CSH, ', ' + e.Manager) AS VARCHAR(1000)),
                e.Manager,
                NodeLevel = r.NodeLevel + 1
            FROM
                dbo.Employee e
                JOIN cte_Recur r
                    ON e.Employee = r.Manager
            WHERE 
                e.Manager IS NOT NULL
            )
    SELECT
        commaSeparatedHierarchy = MAX(r.CSH)
    FROM
        cte_Recur r;
GO 

Sample execution...

SELECT 
    eh.commaSeparatedHierarchy
FROM
    dbo.EmployeeHierarchy('peter') eh;

... and the results...

commaSeparatedHierarchy
------------------------------
Peter, James, Amanda, Charlie