CREATE TABLE a (
  id INT
);

CREATE TABLE b (
  id INT,
  a_id INT,
  sum INT
);

INSERT INTO a SET id=1;
INSERT INTO b SET id=1, a_id=1, sum=2;
INSERT INTO b SET id=2, a_id=1, sum=3;

id -- count(*) -- sum(sum)
1  --  2       --  5

SELECT a.id, tmp.*
FROM a
LEFT JOIN (SELECT b.a_id, COUNT(b.a_id) as `count`, SUM(`sum`) as summa
            FROM b
            GROUP BY b.a_id
) tmp
ON tmp.a_id = a.id