Postgres: Conditional statement in with clause - sql

Question

I am trying to migrate Oracle to Postgres.
Wherever in Oracle we have bulk collect function, I have created plain SQL function:
like
CREATE OR REPLACE FUNCTION get_pub_access_tax(a_person_id mobile_user.id%TYPE)
RETURNS TABLE (tax_id integer) AS
$$
WITH v_pub_tax AS (SELECT tax_id FROM mobile_pub_tax)
select tax_id from v_pub_tax ;
$$ LANGUAGE sql;
But there is a scenario in Oracle where data are fetched on conditional basis: (see below example)
FUNCTION get_st_tax(
a_st_type IN VARCHAR2)
RETURN mmt_array
IS
v_tax_list mmt_array := mmt_array();
BEGIN
IF a_st_type = 'top-stories' THEN
SELECT t.tax_id BULK COLLECT INTO v_tax_list
FROM mobile_st_tax t;
RETURN v_tax_list;
ELSE
SELECT t.tax_id BULK COLLECT INTO v_tax_list
FROM mobile_st_tax t WHERE t.stream_type=a_stream_type;
RETURN v_tax_list;
END IF;
END;
How can we convert above Oracle code to Postgres?
I tried using if condition but getting syntax error(below code).
Below code I have tried executing in Postgres, which gives me syntax error.
CREATE OR REPLACE FUNCTION get_st_tax ( a_st_type IN VARCHAR)
RETURNS TABLE (tax_id integer) AS
$$
IF a_stream_type = 'top-stories' THEN
WITH v_tax_list AS (
SELECT t.tax_id FROM mobile_st_tax t)
SELECT tax_id FROM v_tax_list;
ELSE
WITH v_tax_list AS (
SELECT t.tax_id FROM mobile_st_tax t WHERE t.stream_type=a_stream_type)
SELECT tax_id FROM v_tax_list;
END IF;
$$ LANGUAGE SQL;
Kindly help.

Answer 1

The second function can be done in plain sql:
create function get_st_tax(a_st_type text)
returns int[] as $$
select array_agg(t.tax_id)
from mobile_st_tax t
where
t.stream_type = a_stream_type and a_st_type <> 'top_stories'
or
a_st_type = 'top-stories'
;
$$ language sql;