SELECT first row in recursive part of table expression - sql

Question

I tried to get the first row for a WHERE statement in a recursive table expression. Sadly i'm getting this error:
The TOP or OFFSET operator is not allowed in the recursive part of a recursive common table expression 'cteTree'.
Here's my SQL Query:
WITH cteTree AS(
SELECT
cct.CategoryID AS bla,
ct.CategoryID,ct.ParentCategoryID,
ct.Name,
ct.Published,
CASE
WHEN EXISTS(SELECT ProductID FROM Product WHERE ProductID = (SELECT TOP(1) ProductID FROM ProductCategory WHERE ProductCategory.CategoryID = ct.CategoryID) AND Product.Published = 1) THEN 1
ELSE 0
END AS ProductExists,
1 As Cycle
FROM Category AS ct
LEFT JOIN Category AS cct ON cct.ParentCategoryID = ct.CategoryID
WHERE cct.CategoryID IS NULL
UNION ALL
SELECT
cct.CategoryID AS bla,
ct.CategoryID,
ct.ParentCategoryID,
ct.Name,
ct.Published,
CASE
WHEN cct.ProductExists = 1 THEN 1
WHEN EXISTS(SELECT ProductID FROM Product WHERE ProductID = (SELECT TOP(1) ProductID FROM ProductCategory WHERE ProductCategory.CategoryID = ct.CategoryID) AND Product.Published = 1) THEN 1
ELSE 0
END AS ProductExists,
Cycle + 1
FROM Category AS ct
JOIN cteTree AS cct ON ct.CategoryID = cct.ParentCategoryID
)
SELECT * FROM cteTree
The problem is in the second Case statement under UNION ALL.
SELECT ProductID FROM Product WHERE ProductID = (SELECT TOP(1) ProductID FROM ProductCategory WHERE ProductCategory.CategoryID = ct.CategoryID) AND Product.Published = 1
Does someone know if there's another expression for selecting the first row in a recursive Table expression that works?

Answer 1

There are some tricks to use TOP (1) in a CTE, such as using ROW_NUMBER instead, or putting it into a TVF. But in your case you can just use a normal join:
You should also use NOT EXISTS instead of the LEFT JOIN IS NULL construct.
WITH cteTree AS(
SELECT
cct.CategoryID AS bla,
ct.CategoryID,ct.ParentCategoryID,
ct.Name,
ct.Published,
CASE
WHEN EXISTS (SELECT 1
FROM Product p
JOIN ProductCategory pc ON pc.CategoryID = ct.CategoryID
WHERE p.Published = 1
) THEN 1
ELSE 0
END AS ProductExists,
1 As Cycle
FROM Category AS ct
WHERE NOT EXISTS (SELECT 1
FROM Category AS cct
WHERE cct.ParentCategoryID = ct.CategoryID
)
UNION ALL
SELECT
cct.CategoryID AS bla,
ct.CategoryID,
ct.ParentCategoryID,
ct.Name,
ct.Published,
CASE
WHEN cct.ProductExists = 1 THEN 1
WHEN EXISTS (SELECT 1
FROM Product p
JOIN ProductCategory pc ON pc.CategoryID = ct.CategoryID
WHERE p.Published = 1
) THEN 1
ELSE 0
END AS ProductExists,
Cycle + 1
FROM Category AS ct
JOIN cteTree AS cct ON ct.CategoryID = cct.ParentCategoryID
)
SELECT *
FROM cteTree