Using variables in a plpgsql function - postgresql

Question

Ok so I used a string_agg like this.
select string_agg(DISTINCT first_name,', ' ORDER BY first_name) FROM person_test;
Then I wrote this to return the values to a table.
SELECT *
FROM person_test
where first_name = ANY(string_to_array('Aaron,Anne', ','));
Now I want to put this in a function so that instead of acturally putting names into the string_to_array, I can just call the string_agg.
I am new to postgres and am not finding any good documentation on how to do this online. I believe I would have to declare the the string_agg and then call it in string_to_array but I am having no such luck.
This was my attempt, I know this is now right but if anyone could add some feedback. I am getting an error between results and ALAIS and on the return.
create or REPLACE FUNCTION select_persons(VARIADIC names TEXT[]);
declare results ALIAS select string_agg(DISTINCT first_name,', ' ORDER BY first_name) FROM person_test;
BEGIN
return setof person_test LANGUAGE sql as $$
select * from person_test
where first_name = any(results)
end;
$$ language sql;

Answer 1

You can create a function with variable number of arguments.
Example:
create table person_test (id int, first_name text);
insert into person_test values
(1, 'Ann'), (2, 'Bob'), (3, 'Ben');
create or replace function select_persons(variadic names text[])
returns setof person_test language sql as $$
select *
from person_test
where first_name = any(names)
$$;
select * from select_persons('Ann');
id | first_name
----+------------
1 | Ann
(1 row)
select * from select_persons('Ann', 'Ben', 'Bob');
id | first_name
----+------------
1 | Ann
2 | Bob
3 | Ben
(3 rows)
To use a variable inside a plpgsql function, you should declare the variable and use select ... into (or assignment statement). Example:
create or replace function my_func()
returns setof person_test
language plpgsql as $$
declare
aggregated_names text;
begin
select string_agg(distinct first_name,', ' order by first_name)
into aggregated_names
from person_test;
-- here you can do something using aggregated_names
return query
select *
from person_test
where first_name = any(string_to_array(aggregated_names, ', '));
end $$;
select * from my_func();