Oracle中的UPDATE FROM解决方法

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http://tech.it168.com/db/o/2006-06-28/200606280954219.shtml

 

表的更新操作中,在很多情况下需要在表达式中引用要更新的表以外的数据。象sql server提供了update的from 子句,可以将要更新的表与其它的数据源连接起来。虽然只能对一个表进行更新,但是通过将要更新的表与其它的数据源连接起来,就可以在update的表达式中引用要更新的表以外的其它数据。

    例如:

UPDATE dbo.Table2 SET dbo.Table2.ColB = dbo.Table2.ColB + dbo.Table1.ColB FROM dbo.Table2 INNER JOIN dbo.Table1 ON (dbo.Table2.ColA = dbo.Table1.ColA);

    实际更新的操作是在要更新的表上进行的,而不是在from子句所形成的新的结果集上进行的。

    Oracle没有update from语法,可以通过两种写法实现同样的功能:

    1:子查询UPDATE A SET A.NAME=(SELECT B.NAME FROM B WHERE B.ID=A.ID),本查询要根据具体情况看看是否变通成如下

UPDATE A SET A.NAME=(SELECT B.NAME FROM B WHERE B.ID=A.ID) WHERE A.ID IN (SELECT ID FROM B);

    2:利用视图来做

UPDATE (SELECT A.NAME ANAME,B.NAME BNAME FROM A,B WHERE A.ID=B.ID) SET ANAME=BNAME;

    例如:

UPDATE tablea a SET a.fieldforupdate = (SELECT b.fieldsource FROM tableb b WHERE a.keyfield = b.keyfield) WHERE EXISTS (SELECT b.fieldsource FROM tableb b WHERE a.keyfield = b.keyfield)

    有三点需要注意:

    1. 对于一个给定的a.keyfield的值,SELECT b.fieldsource FROM tableb b WHERE a.keyfield = b.keyfield 的值只能是一个唯一值,不能是多值。

    2. 在绝大多数情况下,最后面的where EXISTS子句是重要的,否则将得到错误的结果。

    3. 对于视图更新的限制:

    如果视图基于多个表的连接,那么用户更新(update)视图记录的能力将受到限制。除非update只涉及一个表且视图列中包含了被更新的表的整个主键,否则不能更新视图的基表。

 
 
 
 
另外一篇外国友人文章:
http://geekswithblogs.net/WillSmith/archive/2008/06/18/oracle-update-with-join-again.aspx
 

Those with a SqlServer background will be familiar with the UPDATE .. FROM syntax. 

For example (totally made up)

   1: update employee_bonus
   2:    set bonus = 0 
   3:   from employee_bonus b 
   4:  inner join employees e on b.employee_id = e.employee_id
   5:  where e.bonus_eligible = 'N'

Those who transitioned from SqlServer to Oracle might find the absence of the UPDATE FROM a significant loss.  The best Oracle alternative that I know of is as follows.

   1: update (select bonus 
   2:           from employee_bonus b 
   3:          inner join employees e on b.employee_id = e.employee_id 
   4:          where e.bonus_eligible = 'N') t
   5:    set t.bonus = 0

Actually, if you look at it, they are very similar.

In fact Oracle has quite a bit of power in the UPDATE.  Check out this reference.  The syntax in the statement above utilizes the subquery1 block in the diagram below.

Happy coding.