• 需求
  有表成绩T1，记录学生成绩，表T2是一张加分的表，现在用表T2给少数名族学生加分。
• 表成绩表T1

PID      score ISMINORITY
---- ---------- ----------
   1        670 N
   2        620 N
   3        600 N
   4        520 Y
   5        480 N
   6        568 Y

• 表T2（加分表）

PID  ADD_score
---- ----------
   3         21
   4        100
   6        100

• 写法1（可能有潜在error）

MERGE INTO T1 USING T2 ON (T1.PID = T2.PID) WHEN MATCHED THEN UPDATE SET T1.score = T1.score + T2.ADD_score WHERE T1.ISMinority = 'Y';

写法1有潜在error是因为Merge 不能多次更新被更新表（即T1）的同一条记录，如果T2有重复的数据，比如两条pid为4的记录。那么写法1将报错
ORA-30926: unable to get a stable set of rows in the source tables
但是如果T2有两条pid为3则不会报错

• 升级版写法

MERGE INTO T1 USING (SELECT PID,MAX(add_score) add_score FROM t2 GROUP BY PID) T2 ON (T1.PID = T2.PID) WHEN MATCHED THEN UPDATE SET T1.score = T1.score + T2.ADD_score WHERE T1.ISMinority = 'Y';