How to return null if id does not exist

For the example table acts below:

and the query below:

select request_id, id
from acts
where id in (234,531,876)

I need to get the following result:

As you could see there is not row where request_id 876 exists. For these cases null should be returned.

How could I achieve this?

>Solution :

Assuming you actually meant to join on request_id not id.

create table acts (id integer, request_id integer);

insert into acts values (1, 234), (2, 531);

select 
   * 
from 
   (values(234), (531), (876)) as t(r_id)  
left join 
    acts on t.r_id  = acts.request_id;

r_id |  id  | request_id 
------+------+------------
  234 |    1 |        234
  531 |    2 |        531
  876 | NULL |       NULL
 

The above creates a VALUES list that you can then LEFT JOIN to acts. In that case the request_id missing in acts will show up with NULL values for the corresponding field in acts.