PostgreSQL add each item to list - DB functions - sql

Question

I'd like to create a DB function that will accept a list of numbers and return a list of numbers. For each item in the list that was passed to the function, it should check some condition and add it to the response list. However, I don't think the way I am trying to do it is really a correct one. What I tried writing is basically some pseudo code here.
CREATE OR REPLACE FUNCTION map_numbers(numbers integer[])
returns integer[]
AS
$BODY$
DECLARE return_list integer[];
FOREACH field IN ARRAY numbers LOOP
CASE
WHEN field = 3 THEN -- add 43 (this was a random thought, but I am basically trying to map a few of the numbers to different values)
END
END LOOP;
RETURN QUERY SELECT * FROM return_list;
$BODY$
LANGUAGE sql VOLATILE
COST 100;

Answer 1

You need to put this into an IF statement. To append a value to an array use ||
CREATE OR REPLACE FUNCTION map_numbers(numbers integer[])
returns integer[]
AS
$BODY$
DECLARE
return_list integer[] := integer[];
BEGIN
FOREACH field IN ARRAY numbers LOOP
if field = 3 then
return_list := return_list || 43;
elsif field = 15 then
return_list := return_list || 42;
else
return_list := return_list || field;
end if;
END LOOP;
return return_list; --<< no SELECT required, just return the variable
END;
$BODY$
LANGUAGE plpgsql --<< you need PL/pgSQL, not SQL for the above
STABLE;
This can also be done using SQL rather than PL/pgSQL which usually is more efficient:
CREATE OR REPLACE FUNCTION map_numbers(numbers integer[])
returns integer[]
AS
$BODY$
select array_agg(field order by idx)
from (
select case
when field = 3 then 43
when field = 15 then 42
else field
end as field,
idx
from unnest(numbers) with ordinality as t(field, idx)
) x;
$BODY$
LANGUAGE sql
STABLE;