Conditional PosgreSQL not null column based on value of another table.

function.sql

CREATE OR REPLACE FUNCTION checkClientType(int) RETURNS varchar as $$
DECLARE
	type varchar;
BEGIN
	SELECT client_type INTO type FROM clients WHERE id = $1;
  
 	RETURN type;
END;
$$ LANGUAGE plpgsql;

table.sql

CREATE TABLE aditional_info (
	id SERIAL NOT NULL UNIQUE,
  	client_id int REFERENCES clients(id) ON DELETE SET NULL ON UPDATE CASCADE,
	rg varchar(10) NOT NULL,
  	issuing_body varchar NOT NULL,
  	birth_date timestamp NOT NULL,
  	municipal_registration varchar,
  	CONSTRAINT not_null_municipal_registration CHECK(CASE WHEN checkClientType(client_id) = 'J' THEN municipal_registration IS NOT NULL END)
);