SQL - Case en el Where

WHERE OrderNumber LIKE
CASE WHEN IsNumeric(@OrderNumber) = 1 THEN
@OrderNumber
ELSE
'%' + @OrderNumber
END

------------------------------------------------------------------------------

WHERE
OrderNumber = CASE
WHEN (IsNumeric(@OrderNumber) = 1)
THEN CONVERT(INT, @OrderNumber)
ELSE -9999 -- Some numeric value that just cannot exist in the column
END
OR
FirstName LIKE CASE
WHEN (IsNumeric(@OrderNumber) = 0)
THEN '%' + @OrderNumber
ELSE ''
END

------------------------------------------------------------------------------

WHERE
IF IsNumeric(@OrderNumber) = 1
OrderNumber = @OrderNumber
ELSE
OrderNumber LIKE '%' + @OrderNumber + '%'