可以使用MERGE语句：

MERGE INTO a
USING b
ON (a.name = b.name)
WHEN MATCHED THEN
  UPDATE
  SET marks = a.marks - b.marks
  WHERE a.marks <> b.marks

其中，对于样本数据：

CREATE TABLE A (Name, Marks) AS
SELECT 'Amit',  23 FROM DUAL UNION ALL
SELECT 'Rahul', 12 FROM DUAL UNION ALL
SELECT 'Gaur',  10 FROM DUAL;

CREATE TABLE b (Name, Marks) AS
SELECT 'Amit',  34 FROM DUAL UNION ALL
SELECT 'Rahul', 10 FROM DUAL UNION ALL
SELECT 'Gaur',  10 FROM DUAL;

然后在MERGE之后，表A包含：
| 标记| MARKS |
| - -----| ------------ |
| -11| -11 |
| 2| 2 |
| 十个| 10 |
fiddle

只需在名称上连接表，然后减去值-

SELECT A.name, CASE WHEN A.Marks <> B.Marks THEN A.Marks - B.Marks ELSE A.Marks END Final_marks
  FROM A
  JOIN B ON A.name = B.name;

UPDATE TableA
SET Marks = A.Marks - B.Marks
FROM TableA A
JOIN TableB B ON A.Name = B.Name
WHERE A.Marks <> B.Marks;