根据条件用一个表的字段，去更新另一个表的字段

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

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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
--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....

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);