create or replace function sp_check_login (p_login varchar)
returns boolean
as $$
declare
	v_login varchar;
	v_violation boolean;
begin
	v_login := p_login;
loop
	--checking table admins
	if exists (select login from admins where admins.login = v_login) then 
	v_violation := true;
		if v_violation = true then 
			raise exception 'login % already exists in table admins', v_login ; 
			exit; 
		else v_violation = false;
		end if;
	end if;
	--checking table customers
	if exists (select login from customers where customers.login = v_login) then 
	v_violation = true;
		if v_violation = true then
			raise exception 'login % already exists in table customers', v_login; 
			exit;
		else v_violation = false;
		end if;
	end if;
	--checking table owners
	if exists (select login from owners where owners.login = v_login) then
	v_violation = true;
	if v_violation = true then
		raise exception 'login % already exists in table owners', v_login; 
		exit;
			else v_violation = false;
		end if;
	end if;
	return v_violation;
end loop;
	
end;
$$language plpgsql;