SQL语句 使用一个表更新另一个表的方法

MERGE INTO T T1  
USING (SELECT '1001' AS a,2 AS b FROM dual where = b > 0) T2  
ON ( T1.a=T2.a)  
WHEN MATCHED THEN 
  UPDATE SET T1.b = T2.b  
WHEN NOT MATCHED THEN   
  INSERT (a,b) VALUES(T2.a,T2.b);