Postgres 13

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create table table1 as select 'a'::text as first_name, 100::numeric as price
1 rows affected

create table table2 as select 'b'::text as first_name, 200::numeric as price
1 rows affected

CREATE or replace FUNCTION sales_price(group_by text) 
RETURNS table(first_name text, price decimal)
AS $$
BEGIN
  IF group_by = 'x' THEN
    return query select t1.first_name, t1.price from table1 t1;
  ELSIF group_by = 'y' THEN
    return query select t2.first_name, t2.price from table2 t2;
  END IF;
END;
$$ LANGUAGE plpgsql IMMUTABLE;
select *
from sales_price('x')


first_name
price


a
100


select *
from sales_price('y')


first_name
price


b
200