PostgreSQL temp function for minimum date - sql

Question

I am trying to create a temp function to avoid duplicating blocks of code in the script. I think by creating a temp function in PostgreSQL I will be able to keep the script succinct but I am unfamiliar with functions on PostgreSQL
I would like to get the minimum date value by a specific group,
i.e.
I have the following SQL script
select
my_tale.some_value_id
, min(my_tale.datetime::date)
filter(
where
my_tale.some_value_id_2 in (1, 2)
) as firs_date
from
my_tale
group by
1
I would like to swap out some_value_id_2 in a function so I don't have to reuse code, i.e. I don't want to do this:
select
my_tale.some_value_id
, min(my_tale.datetime::date)
filter(
where
my_tale.some_value_id_2 in (1, 2)
) as firs_date_1
, min(my_tale.datetime::date)
filter(
where
my_tale.some_value_id_2 in (3, 4)
) as firs_date_2
, min(my_tale.datetime::date)
filter(
where
my_tale.some_value_id_2 in (5, 6)
) as firs_date_3
from
my_tale
group by
1
Ideally I would like to have something like this:
select
my_tale.some_value_id
, temp_function(1,2) as firs_date_1
, temp_function(3,4) as firs_date_2
, temp_function(5,6) as firs_date_3
from
my_tale
group by
1
I have tried playing around with the function below, but can't get it to work:
create function pg_temp.get_first_date(id_1 int, id_2 int)
returns date
language sql
as $function$
min(my_tale.datetime::date)
filter(
where my_tale.some_value_id_2 in ($id_1, $id_2)
);
$function$
;
select
my_tale.some_value_id
, pg_temp.get_first_date(1,2) as firs_date_1
from
my_tale
group by
1
Any advice would be appreciated, thanks in advance

Answer 1

sql fiddle
reference: https://dba.stackexchange.com/questions/300507/what-does-begin-atomic-end-mean-in-a-postgresql-sql-function-procedure
begin;
create temp table mytable(mytableid bigint,some_value_id numeric, some_value_id_2 numeric, datetime date);
insert into mytable values (1,1,1,'2022-01-12');
insert into mytable values (1,1,2,'2022-03-01');
insert into mytable values (1,1,3,'2022-01-01');
insert into mytable values (1,1,1,'2022-01-11');
commit;
create function
pg_temp.get_first_date(id_1 int, id_2 int)
returns date
language sql
BEGIN ATOMIC
select min(datetime)
filter(where mytable.some_value_id_2 in ($1, $2)) from mytable;
end;