CREATE TABLE master
(id        NUMBER PRIMARY KEY,
 name      VARCHAR2(20),
 total     NUMBER
);

CREATE TABLE detail
(id_fk     NUMBER CONSTRAINT fk_dm  
                  REFERENCES master (id),
 sub_total NUMBER
);

detail: id_fk  sub_total
        -----  ---------
        1      10
        1      20
        2      50

master: id     total
        --     -----
        1      30 (= 10 + 20)
        2      50

SELECT m.id, m.name, SUM(d.sub_total) total
FROM MASTER m, DETAIL d
WHERE m.id = d.id_fk
GROUP BY m.id, m.name;