CASE-Statement in WHERE Clause not working what am I missing?

Question

Trying to use a CASE statement in the WHERE clause and it's giving me errors. What did I miss?

SELECT  DISTINCT
        Commodity,
        Commodity_ID, 
        [Description],
        Train,
        Truck
FROM    dbo.List_Commodity
WHERE
        CASE WHEN @Truck = 1 THEN
            Truck = @Truck
        WHEN @Train = 1 THEN
            Train = @Train 
        END

Answer 1

Instead of CASE statement use AND/OR logic to do this

SELECT DISTINCT commodity, 
                commodity_id, 
                [description], 
                train, 
                truck 
FROM   dbo.list_commodity 
WHERE  ( @Truck = 1 
         AND truck = @Truck ) 
        OR ( @Train = 1 
             AND train = @Train )     

In case you want to retrieve all the records when both @Truck and @Train are not equal to 1 then add the below condition to above query

        OR ( @Train <> 1 
             AND @Train <> 1)     

Answer 2

如果您期望THEN truck = @Truck将返回true或false那么您将错过MS SQL没有布尔数据类型和布尔表达式的事实。

Answer 3

If you still want to use the CASE statement, following query can be helpful:

DECLARE @Truck INT = 1  -- 1 is an example
DECLARE @Train INT = 1  -- 1 is an example

SELECT  DISTINCT
        Commodity,
        Commodity_ID, 
        [Description],
        Train,
        Truck
FROM    dbo.List_Commodity
WHERE   1 =
        (
            CASE
                WHEN @Truck = 1 AND Truck = @Truck THEN 1
                WHEN @Train = 1 AND Train = @Train THEN 1
            END
        )

PS: Please note that the CASE statement only returns a value which should be compared it here.