CREATE TABLE tabla1 (
    id INT PRIMARY KEY
);

CREATE TABLE tabla2 (
    id INT PRIMARY KEY
);


INSERT INTO tabla1(id) VALUES(1),(2),(3);

INSERT INTO tabla2(id) VALUES(2),(3),(4);

#union
select id from tabla1
union
select id from tabla2

#intersec
select id from tabla1
where id in (select id from tabla2)

#except
select id from tabla1
where id not in (select id from tabla2)