How to call a recursive function in sql server - 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

Answer 2

I am ashamed, but I used #astander scipt to give string result.
First I created data you gave.
Second I collect rows which I need
And then using XML I put everything in one row (function STUFF removes first comma)
DECLARE #TABLE TABLE(
cat_id INT,
Cat_Name VARCHAR(50),
Main_Cat_Id INT
)
DECLARE #Collected TABLE(
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
INSERT INTO #TABLE SELECT 14,'chinese',6
DECLARE #nodeID INT = 1;
DECLARE #result VARCHAR(MAX);
;WITH Recursives AS (
SELECT cat_id, main_cat_id
FROM #TABLE
WHERE Cat_Id = #nodeID
UNION ALL
SELECT T.cat_id, T.main_cat_id
FROM #TABLE AS T
INNER JOIN Recursives AS R
ON t.Main_Cat_Id = r.cat_id
)
INSERT INTO #Collected
SELECT cat_id
FROM Recursives
SELECT #result = STUFF(
(SELECT ',' + CAST( cat_id AS VARCHAR)
FROM #Collected
ORDER BY cat_id
FOR XML PATH('')
), 1,1,'')
SELECT #result

Answer 3

Your cursor is looping infinitely because you asked it to keep going until 1 no longer equals 1:
WHILE 1 =1
1=1 is always true so the loop never ends, and you don't explicitly break out of it anywhere.
You would do well to study some examples of cursors, for example this one in the Microsoft T-SQL documentation. They are quite formulaic and the main syntax rarely needs to vary much.
The standard approach after opening the cursor is to do an initial fetch next to get the first result, then open a while loop conditional on ##FETCH_STATUS = 0 (0 meaning successful).
Because you're looking only for unsuccessful cursor fetch states inside your cursor:
IF ##FETCH_STATUS <> 0
The setting of #Category will only happen once the cursor has gone past the last row in the set. I suspect this is exactly what you don't want.
I'm also not sure about the scoping of the #Category variable, since it's an input parameter to the function; I generally create new variables inside a function to work with, but off the top of my head I'm not sure this will actually create a problem or not.
More generally, although I don't totally understand what you're trying to achieve here, a recursive function involving a cursor is probably not the right way to do it, as Adriaan Stander's answer suggests.