Always getting back a row, even with no results

Hi List,

I have a simple table:

CREATE TABLE my_table
(
id integer NOT NULL UNIQUE,
cat integer
);

insert into my_table (1, 2);
insert into my_table (2, 2);
insert into my_table (3, 3);
insert into my_table (4, 3);
insert into my_table (5, 2);

I want do a very basic query:
select id from my_table where cat = 3
|id|
|3|
|4|

This will of course return the two rows with that category. But I also want to be able to run the query with a non-existent cat and get a result of "null" for the id.

select id from my_table where cat = 500

would return
|id|
|NULL|

now I can do that with a union all:

select id from my_table where cat = 500
union all
select
NULL as id

|id|
|NULL|

But if I then run that query using a cat value of "3", it will not only return the results, but a third result, of NULL, which I don't want.
|id|
|3|
|4|
|NULL|

I would like to always get a result, either of NULL, or if there are actual results, or those actual results without a NULL if they exist.

I don't see it being possible with any of the coalesce/ifnull/case features, as they only action based on a single row and this can return multiple rows. Maybe something with the window functions or a CTE, but they're both new to me.

Is this possible?

Thanks