How to call a recursive function in sql server

Question

I have a table as follows

cat_id  Cat_Name    Main_Cat_Id

1       veg         null
2       main course 1
3       starter     1
4       Indian      2
5       mexican     2
6       tahi        3
7       chinese     3
8       nonveg      null
9       main course 8
10      indian      9
11      starter     8
12      tahi        11
13      chinese     11

(Main_Cat_Id is cat_id of previously added category in which it belongs)

This table is used for the categories the product where veg category has the two sub category main course and starter which is identify by main_cat_id and those subcategories again has sub category as indian and mexican

And this categorization is dependent on the user; he can add more sub categories to indian, mexican also so that he can have any level of categorization

now I have to select all the subcategories of any node like if I take veg i have to select

(1)veg > (2)main course(1) > (4)indian(2)
                           > (5)mexican(2)
       > (3)starter(1)     > (6)thai(3)
                           > (7)chinese(3)

to form the string as 1,2,4,5,3,6,7

to do this i wrote a sql function as

CREATE FUNCTION [dbo].[GetSubCategory_TEST]
( @MainCategory int, @Category varchar(max))
RETURNS varchar(max)
AS
BEGIN
    IF EXISTS (SELECT Cat_Id FROM Category WHERE Main_Cat_Id=@MainCategory)
    BEGIN
        DECLARE @TEMP TABLE
        (
            CAT_ID INT
        )
        INSERT INTO @TEMP(CAT_ID) SELECT Cat_Id FROM Category WHERE Main_Cat_Id=@MainCategory
        DECLARE @TEMP_CAT_ID INT
        DECLARE CUR_CAT_ID CURSOR FOR SELECT CAT_ID FROM @TEMP
            OPEN CUR_CAT_ID
            WHILE 1 =1
                BEGIN
                FETCH NEXT FROM CUR_CAT_ID
                INTO  @TEMP_CAT_ID;
                IF @@FETCH_STATUS <> 0
                    SET @Category=@Category+','+ CONVERT(VARCHAR(50), @TEMP_CAT_ID)
                    SET @Category = [dbo].[GetSubCategory](@TEMP_CAT_ID,@Category)
                END
                CLOSE CUR_CAT_ID
                DEALLOCATE CUR_CAT_ID
    END
    return @Category
END 

but this function keep on executing and not gives the desired output i don't understands what wrong is going on plz help me to get this

Answer 1

You dont need a recursive function to build this, you can use a Recursive CTE for that.

Something like

DECLARE @TABLE TABLE(
    cat_id INT,
    Cat_Name VARCHAR(50),
    Main_Cat_Id INT
)

INSERT INTO @TABLE SELECT 1,'veg',null
INSERT INTO @TABLE SELECT 2,'main course',1
INSERT INTO @TABLE SELECT 3,'starter',1
INSERT INTO @TABLE SELECT 4,'Indian',2
INSERT INTO @TABLE SELECT 5,'mexican',2
INSERT INTO @TABLE SELECT 6,'tahi',3
INSERT INTO @TABLE SELECT 7,'chinese',3
INSERT INTO @TABLE SELECT 8,'nonveg',null
INSERT INTO @TABLE SELECT 9,'main course',8
INSERT INTO @TABLE SELECT 10,'indian',9
INSERT INTO @TABLE SELECT 11,'starter',8
INSERT INTO @TABLE SELECT 12,'tahi',11
INSERT INTO @TABLE SELECT 13,'chinese',11

;WITH Recursives AS (
        SELECT  *,
                CAST(cat_id AS VARCHAR(MAX)) + '\' ID_Path
        FROM    @TABLE
        WHERE   Main_Cat_Id IS NULL
        UNION ALL
        SELECT  t.*,
        r.ID_Path + CAST(t.cat_id AS VARCHAR(MAX)) + '\'
        FROM    @TABLE t INNER JOIN
                Recursives r    ON  t.Main_Cat_Id = r.cat_id
)
SELECT  *
FROM    Recursives