Postgres 14

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drop table if exists my_table;
create table my_table(id int primary key, first_name text, last_name text);
insert into my_table values
(1, 'John', null),
(2, 'John', 'Frank');
2 rows affected

with condition(do_exist) as (
    select exists(
        select from my_table
        where first_name = 'John'
        and last_name = 'Frank')
)

select id
from my_table 
cross join condition
where first_name = 'John'
and case when do_exist then last_name = 'Frank' else last_name is null end;


id


2


with condition(do_exist) as (
    select exists(
        select from my_table
        where first_name = 'John'
        and last_name = 'Jonas')
)

select id
from my_table 
cross join condition
where first_name = 'John'
and case when do_exist then last_name = 'Jonas' else last_name is null end;


id


1