update tab1 B set b=b-(select max(b) from tab1 A where A.b=A.c and A.a=010400) ,
c=c-(select max(c) from tab1 A where A.b=A.c and A.a=010400) 
where a=010000;

update tab1 B set b=b-(select max(b) from tab1 A where A.b=A.c and A.a=010400) ,
c=c-(select max(c) from tab1 A where A.b=A.c and A.a=010400) 
where a=000000;

update tab1 B set b=0 ,c=0
where a=000000 and b=c;

------解决方案--------------------
实测数据
SQL code

CREATE TABLE T134
(
    a VARCHAR2(20),
    b NUMBER(4),
    c NUMBER(4),
    d VARCHAR2(20)
);


INSERT INTO T134 VALUES('000000', 20, 20, 'a');
INSERT INTO T134 VALUES('010000', 15, 15, 'a');
INSERT INTO T134 VALUES('010400', 5, 5, 'a');

INSERT INTO T134 VALUES('000000', 25, 25, 'b');
INSERT INTO T134 VALUES('010000', 15, 15, 'b');
INSERT INTO T134 VALUES('010400', 5, 5, 'b');

INSERT INTO T134 VALUES('000000', 20, 20, 'c');
INSERT INTO T134 VALUES('010000', 15, 15, 'c');
INSERT INTO T134 VALUES('010400', 5, 0, 'c');

-- 更新a = '0000000' 和a='010000'的行
UPDATE T134 T1 SET b = b - (SELECT b FROM T134 T2 WHERE b = c AND a = '010400' AND T2.d = T1.d),
                c = c - (SELECT c FROM T134 T3 WHERE b = c AND a = '010400' AND T3.d = T1.d)
WHERE (a = '000000' OR a = '010000') AND 
EXISTS(SELECT 1 FROM T134 T4 WHERE b = c AND a = '010400' AND T4.d = T1.d)
-- 更新 a = '010400'的行
UPDATE T134 SET b = 0, c = 0 WHERE b = c AND a = '010400';