Where Am I Doing this wrong? TSQL IF Clause

CREATE PROCEDURE SP_TEST(@SellStartDate datetime = NULL,@SellEndDate datetime = NULL)
AS
BEGIN
IF (@SellEndDate IS NULL AND @SellStartDate IS NOT NULL)
        SELECT [ProductID], [Name],[ProductNumber]      FROM [AdventureWorks2012].[production].[Product]
        WHERE SellStartDate = @SellStartDate 
ELSE IF (@SellStartDate IS NULL AND @SellEndDate IS NOT NULL)
                        SELECT [ProductID], [Name],[ProductNumber]
                        FROM [AdventureWorks2012].[production].[Product]
                        WHERE SellEndDate = @SellEndDate 
         ELSE
                IF (@SellStartDate IS NOT NULL AND @SellEndDate  IS NOT NULL)  
                        SELECT [ProductID], [Name],[ProductNumber]
                        FROM [AdventureWorks2012].[production].[Product]
                        WHERE SellStartDate >= @SellStartDate AND SellEndDate <= @SellEndDate 
END

TRY THIS.

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Regards, RSingh