CREATE TABLE t(k, a);
CREATE TABLE u(k, b);

INSERT INTO t(k, a) VALUES (1, 10), (2, 20), (3, 30);

SELECT
  ifnull(sum(a), 0) as 'sum(a)',
  ifnull(sum(b), 0) as 'sum(b)'
  FROM t FULL OUTER JOIN u ON u.k = t.k;