How does one use WITH within a Scalar Function? - tsql

Question

I'm trying to add a WITH clause to a statement within a scalar function but when I do, I get the following syntax errors:
SQL80001: Incorrect syntax near 'WITH'
SQL80001: Incorrect syntax near ')'
The last error refers to the final closing bracket.
Here a greatly-simplified example of the SQL which also fails for the same reason:
CREATE FUNCTION IsSumEqualToTen
(
#number1 INT,
#number2 INT
)
RETURNS BIT
AS
BEGIN
RETURN
(
WITH AddNumbers AS
(
SELECT #number1 + #number2
)
SELECT CASE WHEN AddNumbers = 10
THEN 1
ELSE 0
END
)
END
I know that when changed to this, it has no syntax errors:
CREATE FUNCTION IsSumEqualToTen
(
#number1 INT,
#number2 INT
)
RETURNS BIT
AS
BEGIN
RETURN
(
CASE WHEN (#number1 + #number2) = 10
THEN 1
ELSE 0
END
)
END
But for what I'm actually doing I need a recursive CTE to flatten a heirarchy so the WITH is essential.

Answer 1

First, I must say I agree with Larnu on this one - using a scalar function is probably not the best course of action you can take.
However, I wanted to show you that there's no problem using a common table expression in a user defined function - table valued or scalar.
The problem with the code you've shown is that you do have some syntax errors as well as some conceptual errors. A fixed version of that function (still using a cte) is this:
CREATE FUNCTION IsSumEqualToTen
(
#number1 INT,
#number2 INT
)
RETURNS BIT
AS
BEGIN
DECLARE #Result bit;
WITH AddNumbers(AddNumbers) AS
(
SELECT #number1 + #number2
)
SELECT #Result = CASE WHEN MAX(AddNumbers) = 10
THEN 1
ELSE 0
END
FROM AddNumberes;
RETURN #Result;
END
Notes:
Each column in the cte result set must be named.
A cte is treated as a table, you must take into consideration the fact that it might contain more than one row (even if it can only contain one, the compiler can't know that).
scalar functions are not inlined, this means that you don't do return (body here) but body...; return scalarValue. Infact, this is how the official documentation looks like:
CREATE [ OR ALTER ] FUNCTION [ schema_name. ] function_name
( [ { #parameter_name [ AS ][ type_schema_name. ] parameter_data_type
[ = default ] [ READONLY ] }
[ ,...n ]
]
)
RETURNS return_data_type
[ WITH <function_option> [ ,...n ] ]
[ AS ]
BEGIN
function_body
RETURN scalar_expression
END
[ ; ]