postgres: Using trigger function to remove user from a group - postgresql

Question

I have a trigger function that gets a person's username from their first and last name, and then drops the user with that username from a group with privileges, essentially locking that person out of the database.
returns "trigger" AS'
DECLARE
uname varchar(255);
begin
uname = (SELECT CONCAT(LOWER(first_name), LOWER(last_name)) FROM members WHERE library_card_id = (SELECT members_library_card_id FROM borrowed_books WHERE fine_id IS NOT NULL));
alter group members drop user uname;
end;
' LANGUAGE 'plpgsql';
CREATE TRIGGER block_member_if_fine
AFTER UPDATE on borrowed_books
FOR EACH ROW
EXECUTE PROCEDURE f_block_member_if_fine();
However, when I use the trigger function, I get an error role "uname" does not exist
Is there a way to use uname as a variable? Or maybe there's a different way of dropping a user with a trigger function?

Answer 1

Try this :
CREATE OR REPLACE PROCEDURE f_block_member_if_fine()
RETURNS "trigger" LANGUAGE 'plpgsql' AS
$$
DECLARE
uname varchar(255);
BEGIN
SELECT CONCAT(LOWER(m.first_name), LOWER(m.last_name))
INTO uname
FROM members AS m
WHERE m.library_card_id = NEW.members_library_card_id
AND NEW.fine_id IS NOT NULL;
IF FOUND
THEN
EXECUTE 'ALTER GROUP members DROP USER '|| quote_nullable(uname) ;
END IF ;
RETURN NEW ;
END ;
$$
CREATE TRIGGER block_member_if_fine
AFTER UPDATE on borrowed_books
FOR EACH ROW
EXECUTE PROCEDURE f_block_member_if_fine();