注意ON关联值不能重复。

SELECT A.A1,A.A2,B.B1,B2 FROM TABLEA ALEFT JOIN TABLEB BON A.A1 = B.B1WHERE A.A2 IS NULL AND B.B2 IS NOT NULL------------------------------------MERGE INTO TABLEA AUSING TABLEB  B ON (A.A1 = B.B1 ) WHEN MATCHED AND A.A2 IS NULL AND B.B2 IS NOT NULL THEN UPDATE SET A.A2=B.B2 --这个比UPDATE的好处就是只全表扫描一次，还可以一次性做多个操作，如UPDATE,INSTER,DELETE--WHEN NOT MATCHED THEN--INSERT....

数据量少的情况下，容易理解的

UPDATE A a  SET(A1) = (SELECT b.B1 FROM B b where  a.ID=b.ID)WHERE EXISTS (SELECT 1 FROM B b WHERE a.ID = b.ID);