CREATE TABLE t(a, b, x);
CREATE TABLE u(a, b, y);
INSERT INTO t(a, b, x) VALUES (1, 2, 10);
INSERT INTO u(a, b, y) VALUES (1, 2, 20);
SELECT x, y
  FROM t
       INNER JOIN u ON TRUE
           AND t.a = u.a
           AND t.b = u.b