当需要更新某张表，但数据来自另一张表时

关键点：

 - Update后面一定要是真实表名

 - From后面一定要是另外的表

 - Where部分，1.要有需要更新的真实表与数据来源各表之前的关联条件；2.需要更新真实表的过滤条件

 

INSERT into a([KEY],VALUE) values(1,100);

INSERT into a([KEY],VALUE) values(2,200);

INSERT into a([KEY],VALUE) values(3,300);

INSERT into b([KEY],VALUE) values(1,100);

INSERT into b([KEY],VALUE) values(2,200);

INSERT into b([KEY],VALUE) values(3,300);

SELECT * FROM a;

SELECT * FROM b;

DELETE FROM a;

DELETE FROM b;

update a SET [VALUE] = a.[VALUE] + t2.[VALUE] FROM (SELECT * FROM b) t2 WHERE a.[Key] = t2.[Key] AND a.[Key] > 1;