SQL Query with condition in where clause

Question

I have written a sql query like,

DECLARE @TypeID BIGINT

--SET @TypeID=169

SELECT TypeID from TrnChecklist where TypeID IN 
(CASE WHEN (@TypeID IS NOT NULL AND @TypeID <> '') THEN @TypeID 
ELSE (select distinct TypeID from TrnChecklist where AuditID=57) END)

If I provide @TypeID it gives me records. But If I do not provide @TypeID then it gives me error

"Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=,......"

But I am using "TypeID IN " in where condition since "select distinct TypeID from TrnChecklist where AuditID=57" returns more than one value

My data sample (in TrnChecklist table. Few other columns but i am using only these 2)

Answer 1

CASE expression evaluates a list of conditions and returns one of multiple possible result expressions.

It does not work because you are returning multiple values per evaluation in a CASE expression which is expected to return a scalar value per evaluation.

If you put case over table data, it evaluates each row to return a value per row .

You can use this query here

SELECT TypeID FROM TrnChecklist WHERE TypeID IN 
(SELECT TypeID FROM TrnChecklist 
WHERE AuditID=57 AND (@TypeID IS NULL OR @TypeID = '') 
UNION SELECT @TypeID)

Answer 2

I Trying to use a case expression inside IN() is not likely to work. Try to use boolean logic. You want the nominated value by parameter, or you want an IN() list match.

DECLARE @TypeID bigint

--SET @TypeID=169

SELECT
  TypeID
FROM TrnChecklist
WHERE (TypeID = @TypeID AND @TypeID IS NOT NULL AND @TypeID <> '') 
OR ( NOT  (TypeID = @TypeID AND @TypeID IS NOT NULL AND @TypeID <> '') 
    AND TypeID IN  (SELECT DTypeID
                    FROM TrnChecklist
                    WHERE AuditID = 57)
   )

There may be a few conditions that can still be removed from the query above but I think it would work.

Note, I have removed distinct. Often the cost of distinct is greater than the size of the in list. Put it back if really needed.