Only one expression can be specified when attempting to use select lists in scalar function

Question

The following procedure sets a boolean value based on the rowcounts of each of the following queries and it is then supposed to return the true(1) or false(0) value back to Access; I am currently getting the error of "Only one expression can be specified in the select list when the subquery is not introduced with EXISTS." How can I get this to work?

 CREATE FUNCTION [dbo].[FN_chk](@pF NVARCHAR(255), @pI NVARCHAR(100), 
    @pL NVARCHAR(100), @pLY NVARCHAR(100))

RETURNS INT AS
BEGIN

    DECLARE @Bool BIT
    SET @Bool = 0

    -- check allDataTemp
    If ((SELECT DISTINCT theFN, theIN, theLN, theANA FROM aDT
        WHERE theFN = @pF  and theIN = @pI 
        and theLN = @pL  and theANA = @pLY) > 0)
        BEGIN
            SET @Bool = 1
            RETURN @Bool
        END 

    -- check allDataTemp2
    If ((SELECT theFN, theIN, theLN, theANA FROM aDT2
        GROUP BY theFN, theIN, theLN, theANA 
        HAVING theFN = @pF  and theIN = @pI 
        and theLN = @pL  and theANA = @pLY) > 0)
        BEGIN
            SET @Bool = 1
            RETURN @Bool
        END 

    -- Return the result of the function
    RETURN @Bool

END

Answer 1

As the ERROR already explains, use EXISTS

 If (EXISTS(SELECT DISTINCT theFN FROM aDT
    WHERE theFN = @pF  and theIN = @pI 
       and theLN = @pL  and theANA = @pLY))
    BEGIN
        SET @Bool = 1
        RETURN @Bool
    END

Similarly change the other IF too...
EDIT: EXISTS is used when the number of rows could be zero. Also if you SELECT just one value to compare, it should work too.

Answer 2

Your query inside the IF returns a large number of columns. Comparing this to a number is meaningless. (eg is "Coding", "Maniac" > 0?)

The quickest fix is to switch to using exists

If exists (SELECT DISTINCT theFN, theIN, theLN, theANA FROM aDT
    WHERE theFN = @pF  and theIN = @pI 
    and theLN = @pL  and theANA = @pLY)
    BEGIN

More optimally, don't both with the columns at all - all you are about is that there are rows:

If exists (SELECT count(1) FROM aDT
    WHERE theFN = @pF  and theIN = @pI 
    and theLN = @pL  and theANA = @pLY) 
    BEGIN