oracle merge 目标表以及源表存在重复列的问题（转）

SQL> select * from t_source;                                                                                             
                                                                                                                         
                                      A                                       B                                          
--------------------------------------- ---------------------------------------                                          
                                      1                                       1                                          
                                      2                                       2                                          
                                                                                                                         
SQL> select * from t_target;                                                                                             
                                                                                                                         
                                      A                                       B                                          
--------------------------------------- ---------------------------------------                                          
                                      2                                      34                                          
                                      2                                    3999                                          
                                                                                                                         
SQL> merge into t_target using t_source on (t_source.a=t_target.a) when matched then update set t_target.b=t_source.b;   
                                                                                                                         
Done                                                                                                                     
                                                                                                                         
SQL> select * from t_target;                                                                                             
                                                                                                                         
                                      A                                       B                                          
--------------------------------------- ---------------------------------------                                          
                                      2                                       2                                          
                                      2                                       2                                          
                                                                                                                         
SQL> commit;                                                                                                             
                                                                                                                         
Commit complete                                                                                                          
                                                                                                                         
小结：1,对于merge,目标表如果匹配列重复，但源表匹配不重复，会用源表表列新目标表的数据                                     
      2,反之，如果源表匹配列重复，此时merge就会报错，因为不知用源表哪条记录来更新目标表的数据

转自

http://blog.itpub.net/9240380/viewspace-750968/