Select all records where function is false - sql

Question

I have a validation function that I'd like to run on each id in a table, and return only those ids which are invalid. Code that I've tried:
select myID
from myDB.dbo.myTable
where (myDB.dbo.validateID(myID) = 0)
Where myDB.dbo.validateID is a scalar-valued function.
This works but returns null for all the valid IDs - I want to only return the invalid ones. What is the most efficient way to return all the invalid rows using this function?
Update:
The validateID function returns 1 if the ID is valid, 0 if it isn't.
My code above returns null if the ID is valid, and the ID if it's not. I want it to instead only return the invalid IDs, without all the null results.

Answer 1

select myID
from myDB.dbo.myTable
where (myDB.dbo.validateID(myID) = 0)
This works but returns null for all the valid IDs
That is simply not possible. If you select myID, you get myID, so if you get null, then myID must be null, and your validateID function is detecting that as invalid.
If your function should treat null as a valid ID, you need to fix your function so that myDB.dbo.validateID(null) returns 1.
If your function should treat null as neither a valid nor an invalid ID, you need to fix your function so that myDB.dbo.validateID(null) returns null.
If your function should treat null as an invalid ID, but you still want to exclude null results, when just add a condition myID is not null to your selection.

Answer 2

I tried the same logic but am getting proper output as expected. Am getting rows returned only when function outputs 0 (ie) Invalid records. I don't see NULL values for valid records.
create table dbo.Contracts1(id int,name varchar(50),pric int)
INSERT INTO dbo.Contracts1
VALUES ( 1,'invalid',40000),
(2,'valid',50000),
(3,'valid',35000),
(4,'invalid',40000)
CREATE FUNCTION Testt(#id INT)
returns INT
AS
BEGIN
DECLARE #ret INT
IF EXISTS (SELECT 1
FROM dbo.Contracts1
WHERE name = 'Valid'
AND id = #id)
SELECT #ret = 1
ELSE
SELECT #ret = 0
RETURN #ret
END
SELECT *
FROM dbo.Contracts1
WHERE dbo.Testt(id) = 0
OUTPUT
id name pric
-- ------- -----
1 invalid 40000
4 invalid 40000