Passing a nullable local variable to a where condition - sql

Question

I have the following SQL function:
CREATE or ALTER FUNCTION [dbo].[GET_BOOK_CODE_BY_AUTHOR_ID]
(
#AUTHOR_ID int
)
RETURNS varchar(50)
AS
BEGIN
DECLARE
#MOT_N_ID int,
#SCO_N_ID int,
#BOOK_CODE varchar(50);
select
#MOT_N_ID = AUT.MOT_N_ID,
#SCO_N_ID = AUT.SCO_N_ID
from AUTHOR AUT
where AUT.AUT_ID = #AUTHOR_ID
SELECT
#BOOK_CODE = (
select BOO.BOO_CH_CODE
from BOOK BOO
where
and BOO.MOT_N_ID = #MOT_N_ID
and BOO.SCO_N_ID = #SCO_N_ID
)
RETURN #BOOK_CODE
END;
GO
The variable #SCO_N_ID can be a null value however and when it returns a null value SQL interprets the condition as:
BOO.SCO_N_ID = null
It is not interpreting it as:
BOO.SCO_N_ID is null
Any idea of how to do this?

Answer 1

I would do this check
"the variable is null or the field is equal to the variable"
and (#SCO_N_ID is null or BOO.SCO_N_ID = #SCO_N_ID)

Answer 2

How about something like this:
CREATE or ALTER FUNCTION [dbo].[GET_BOOK_CODE_BY_AUTHOR_ID]
(
#AUTHOR_ID int
)
RETURNS varchar(50)
AS
BEGIN
DECLARE
#BOOK_CODE varchar(50);
SELECT TOP 1 #BOOK_CODE = VEN_CH_CODE
FROM BOOK BOO
INNER JOIN AUTHOR AUT
ON AUT.MOT_N_ID = BOO.MOT_N_ID
WHERE
BOO.SCO_N_ID = AUT.SCO_N_ID OR
(BOO.SCO_N_ID IS NULL AND AUT.SCO_N_ID IS NULL)
ORDER BY BOO.SCO_N_ID, BOO.MOT_N_ID
RETURN #BOOK_CODE
END;
Note I have joined the tables to make one query, also note I have put a TOP 1 because it might be possible for the query to return more than one result set. If you use TOP 1 then you should have an ORDER BY clause to make sure you return the result you want

Answer 3

There are a few important points to note here:
Firstly, to compare while taking nulls into account, instead of a = b you can use this syntax exists (select a intersect select b). This will compile down to an IS comparison, as shown here, and is very efficient
You can dispense with all the variables and just do a simple join
User-defined scalar functions are very slow. You will get much better performance from an inline Table-Valued Function
CREATE or ALTER FUNCTION [dbo].[GET_BOOK_CODE_BY_AUTHOR_ID]
(
#AUTHOR_ID int
)
RETURNS TABLE
AS RETURN (
select VEN.VEN_CH_CODE
from BOOK BOO
join AUTHOR AUT on BOO.MOT_N_ID = AUT.MOT_N_ID
and exists (select BOO.SCO_N_ID
intersect
select AUT.SCO_N_ID)
where AUT.AUT_ID = #AUTHOR_ID
);
GO
You can use it like this
SELECT *
FROM GET_BOOK_CODE_BY_AUTHOR_ID(101) b;
Or like this
SELECT *
FROM OtherTable t
CROSS APPLY GET_BOOK_CODE_BY_AUTHOR_ID(t.authorId) b;
Or this
SELECT *,
(SELECT * FROM GET_BOOK_CODE_BY_AUTHOR_ID(t.authorId))
FROM OtherTable t;

Answer 4

I like to set both sides to an unavailable code, I will use -99 in this case.
ISNULL(BOO.SCO_N_ID,-99) = ISNULL(#SCO_N_ID,-99)