MERGE INTO emp t1
USING  (
             select count(*) c from emp t2 where t2.empno='2000' )x
ON (x.c > 0)
when matched then 
      update set t1.job='c++' where t1.empno='1000'
when not matched then 
      insert values('3000','zhangs','c',777,sysdate,9000,2,10);