SQL CASE表达式返回基于null和not null的多种模式的集合

Question

i have task which takes a parameter and has three modes of results

Example Parameter name is sizePram (1 = only null , 2 = only not null, 3 = both or all)

declare @sizeParam int
set @sizeParam = 1 or 2 or 3

select * from SalesLT.Product where Size =
CASE @sizeParam
    when 1 then is null
    when 2 then is not null
    when 3 then all
end

Answer 1

select * 
from SalesLT.Product 
where 
    (@sizeParam = 1 and Size is null) or
    (@sizeParam = 2 and Size is not null) or
    (@sizeParam = 3)

Answer 2

SELECT *
    FROM SalesLT.Product
    WHERE (@sizeParam = 1 AND Size IS NULL)
       OR (@sizeParam = 2 AND Size IS NOT NULL)
       OR (@sizeParam = 3)

Answer 3

SELECT *
  FROM SalesLT.Product
 WHERE ( sizeParam <> 1 OR Size IS NULL )
       AND NOT ( sizeParam = 2 AND Size IS NULL );