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

<pre name="code" class="sql">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。