oracle中merge into..using..on..when..when..用法

语法：

MERGE INTO [your table-name] [rename your table here]
USING ( [write your query here] )[rename your query-sql and using just like a table]
ON ([conditional expression here] AND [...]...)
WHEN MATHED THEN [here you can execute some update sql or something else ]
WHEN NOT MATHED THEN [execute something else here ! ]

实例：

merge into tfa_alarm_act_nms a     
using (select FP0,FP1,FP2,FP3,redefine_SEVERITY     
from tfa_alarm_status) b     
on (a.fp0=b.fp0 and a.fp1=b.fp1 and a.fp2=b.fp2 and a.fp3=b.fp3)     
when matched then update set a.redefine_severity=b.redefine_severity     
when not matched then insert (a.fp0,a.fp1,a.fp2,a.fp3,a.org_severity,a.redefine_severity,a.event_time,a.int_id)     
values (b.fp0,b.fp1,b.fp2,b.fp3,b.redefine_SEVERITY,b.redefine_severity,sysdate,7777778); 

作用：利用表 tfa_alarm_status跟新表tfa_alarm_act_nms 的b.redefine_severity，条件是a.fp0=b.fp0 and a.fp1=b.fp1 and a.fp2=b.fp2 and a.fp3=b.fp3，如果tfa_alarm_act_nms表中没有该条件的数据就插入。


如果你的数据量很大，此sql效率非常高。

转自：http://azrael6619.iteye.com/blog/759678