根據條件用一個表的字段，去更新另一個表的字段

注意ON關聯值不能重復。

SELECT A.A1,A.A2,B.B1,B2 FROM TABLEA A
LEFT JOIN TABLEB B
ON A.A1 = B.B1
WHERE A.A2 IS NULL AND B.B2 IS NOT NULL

------------------------------------

MERGE INTO TABLEA A
USING 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 
--這個比UPDATE的好處就是只全表掃描一次，還可以一次性做多個操作，如UPDATE,INSTER,DELETE
--WHEN NOT MATCHED THEN
--INSERT....

數據量少的情況下，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);