一条SQL的疑问

今天遇到了一个问题，查了一下SQL的执行顺序。http://www.blogjava.net/youling/archive/2009/03/06/243177.html
但是，还是没有解决问题。SQL如下：

update vip set lastlevelmodifieddate=b.adjustdate from vip a, (
select vipno, min(adjustdate) as adjustdate from vipleveladjustmentform
where currentlevel = '1' and lastlevel = '6' and
vipno in (select vipno from vip where joindate < '2010-03-10') group by vipno
)b
where a.vipno=b.vipno

最后，执行的结果居然把在b中不存在的vipno也修改了，不知道为什么？
希望以后找个机会解决，或者有高手也帮忙解决一下。当然，不一定要一模一样的数据库表，大家可以建立两个表，根据相同的原理试一下。例如下面两个表a,b.
CREATE TABLE a
(
  pk integer NOT NULL,
  vipno text,
  createddate timestamp with time zone,
  CONSTRAINT a_pkey PRIMARY KEY (pk)
)
WITHOUT OIDS;
ALTER TABLE a OWNER TO postgres;

CREATE TABLE b
(
  pk integer NOT NULL,
  vipno text,
  createddate timestamp with time zone,
  CONSTRAINT b_pkey PRIMARY KEY (pk)
)
WITHOUT OIDS;
ALTER TABLE b OWNER TO postgres;

然后插入数据，如下：
insert into a values (1, 'a001', '2010-01-05 19:53:36.208')
insert into a values (2, 'a002', '2010-01-05 19:53:36.208')

insert into b values (1, 'a001', '2010-01-01 19:53:36.208')

然后执行SQL：
update a set createddate=c.adjustdate from a as d, (
select vipno, min(createddate) as adjustdate from b group by vipno
)c
where d.vipno=c.vipno
结果表a中的两条数据的createddate都被修改了，为什么呢？

评论

1 楼 laoshifu 2010-03-04  

经过一个多小时的尝试，发现了问题，发现update的表如果在from语句中使用了别名，貌似已经跟要update的表分离开了。例子中的SQL应该如下：
update a set createddate=c.adjustdate from (
select vipno, min(createddate) as adjustdate from b group by vipno
)c
where a.vipno=c.vipno

本来的问题的SQL应该如下：
update vip set lastlevelmodifieddate=b.adjustdate from (
select vipno, min(adjustdate) as adjustdate from vipleveladjustmentform
where currentlevel = '1' and lastlevel = '6' and
vipno in (select vipno from vip where joindate < '2010-03-10') group by vipno
)b
where vip.vipno=b.vipno