SQL> select * from t_order;
 
                               ORDER_ID                                  PAY_ID MODI_DATE
--------------------------------------- --------------------------------------- -----------
                                      1                                        
                                      2                                       2
 
SQL> merge into t_order using t_pay_info on (t_order.order_id=t_pay_info.order_id) when matched then update set t_order.pay_id=t_pay_info.pay_id where t_order.pay_id is null;
 
Done
 
SQL> select * from t_order;
 
                               ORDER_ID                                  PAY_ID MODI_DATE
--------------------------------------- --------------------------------------- -----------
                                      1                                       1
                                      2                                       2
 
SQL> select * from t_order;
 
                               ORDER_ID                                  PAY_ID MODI_DATE
--------------------------------------- --------------------------------------- -----------
                                      1                                        
                                      2                                       2
 
SQL> merge into t_order using t_pay_info on (t_order.order_id=t_pay_info.order_id) when matched then update set t_order.pay_id=t_pay_info.pay_id,t_order.modi_date=sysdate where t_order.pay_id is null;
 
Done
 
SQL> select * from t_order;
 
                               ORDER_ID                                  PAY_ID MODI_DATE
--------------------------------------- --------------------------------------- -----------
                                      1                                       1 2012/12/11
                                      2                                       2
 
SQL>