CREATE TABLE A (id int);
INSERT INTO A VALUES (1),(2),(3),(4),(5);
CREATE TABLE B (id serial, A_id int);
INSERT INTO B (A_id) VALUES (1),(2),(2),(4),(4);
SELECT *
FROM A
WHERE NOT EXISTS (
SELECT true FROM B WHERE A_id = A.id
);
SELECT *
FROM B
RIGTH JOIN A ON A.id = b.A_id;