Select inside a where statement

Question

I just want to make sure I am doing this right.

I have two tables:

Customer:
ID: GUID
name: varchar
... etc
 and Reference
ID: GUID
customerid: GUID ----> FK
Myreference: varchar (max)
timestamp: datetime

I have the following stored procedure which check the reference table if the reference is already being used by another customers.

    @CustomerId [uniqueidentifier], 
    @Myreference [nvarchar]
AS 
BEGIN 
    SELECT  
        CAST(CASE WHEN COUNT(*) > 0 THEN 1 ELSE 0 END AS BIT) AS IsInUse
    FROM  Reference
    WHERE CustomerId <> @CustomerId AND Myreference= @Myreference 
END 
GO 

Now, I need to change the stored procedure to check if the reference is used by another customer to check if that other customer has another reference record then return 0 otherwise return 1.

Here is my change, but I am pretty sure it can be better.

IF (SELECT CAST(CASE WHEN COUNT(*) > 0 THEN 1 ELSE 0 END AS BIT) AS IsInUse
   FROM Reference 
   WHERE CustomerId <> @CustomerId AND  
         Myreference = @Myreference ) = 1 
    BEGIN 
        SELECT CAST(CASE WHEN COUNT(*) > 1 THEN 0 ELSE 1 END AS BIT) AS IsFine 
        FROM Reference 
        WHERE CustomerId = (SELECT CustomerId 
                            FROM Purchase.BureauCreditEnquiry 
                            WHERE CustomerId <> @CustomerId AND  
                                  Myreference = @Myreference ); 
    END; 
ELSE 
    BEGIN 
        SELECT 0; 
    END; 

Answer 1

since this is stored procedure , use sql variables.

DECLARE @IsInUse INT
DECLARE @isFine INT

SET @IsInUse = SELECT CAST(CASE WHEN COUNT(*) > 0 THEN 1 ELSE 0 END AS BIT) AS IsInUse
            FROM Reference 
            WHERE CustomerId <> @CustomerId AND  
            Myreference = @Myreference

SET @isFine = SELECT CAST(CASE WHEN COUNT(*) > 1 THEN 0 ELSE 1 END AS BIT) AS IsFine 
            FROM Reference 
            WHERE CustomerId = (SELECT CustomerId 
                            FROM Purchase.BureauCreditEnquiry 
                            WHERE CustomerId <> @CustomerId AND  
                                  Myreference = @Myreference )

SELECT IIF(@IsInUse >= 1, @isFine, 0)