[转]oracle update set select from 关联更新

本文转自：http://blog.csdn.net/disiwei1012/article/details/52589181

http://www.blogjava.net/Jhonney/archive/2010/06/25/324503.html

$ sqlplus user/pass 
 
SQL*Plus: Release 9.2.0.6. - Production on Wed Aug  ::  
 
Copyright (c) , , Oracle Corporation.  All rights reserved. 
 
 Connected to:
 Oracle9i Enterprise Edition Release 9.2.0.6. - 64bit Production
 With the Partitioning, OLAP and Oracle Data Mining options
 JServer Release 9.2.0.6. - Production 
 
SQL> select * from wwm2;        --要更新的表 
 
TOWN                         ID
 -------------------- ----------
 
 ww'jj                       111
 llll
 dddd
 lllldf
 lllldf
 dsafdf                      
 
 ljjjjj
 dsafdf                      
 
TOWN                         ID
 -------------------- ----------
 
 ljjjjj                      
 
 SQL> select * from wwm5;            --更新的条件表
 
TOWN                         ID
 -------------------- ----------
 lllldf
 test                       
 
SQL> select wwm2.* from wwm2,wwm5 where wwm2.id=wwm5.id
     /
 
TOWN                         ID
 -------------------- ----------
 
 ww'jj                       111
 lllldf
 lllldf
 dsafdf                      
 
 dsafdf                      
 
 rows selected.
 
所以,每次需要更新8条数据就是正确的.
 
相信程序员是通过以下类似的SQL更新的,这是错误的,因为没有加WHERE
 SQL>  update wwm2 set wwm2.town=(select wwm5.town from wwm5 where wwm5.id=wwm2.id)
     /
 
 rows updated.
 
SQL> select * from wwm2;
 
TOWN                         ID
 -------------------- ----------
 
 lllldf
 lllldf                      
 
 lllldf
 lllldf
 lllldf
 lllldf                      
 
 lllldf                      
 
TOWN                         ID
 -------------------- ----------
 lllldf                      
 
  rows selected.
 
可以看到13条记录被更新,符合条件的更新正确,不符合条件的也更新为NULL.以下是正确的方法
 
方法一:
SQL> update wwm2
     set town=(select town from wwm5 where wwm5.id=wwm2.id)
     where id=(select wwm5.id from wwm5 where wwm5.id=wwm2.id)
     /
 
 rows updated.
 
方法二:    与方法一道理相同,这里需要掌握EXIST的相关用法.
SQL> update wwm2
    set town=(select town from wwm5 where wwm5.id=wwm2.id)
    where exists (select  from wwm5 where wwm5.id=wwm2.id)
  rows updated.
 
方法三:
SQL> update (select a.town atown,a.id aid,b.town btown,b.id bid from wwm2 a,wwm5 b where a.id=b.id)
     set atown=btown
     /
 set atown=btown
     *
 ERROR at line :
 ORA-: cannot modify a column which maps to a non key-preserved table
 
  * alter table wwm5 add primary key (id)
 SQL> /
 
Table altered.
 
    update (select a.town atown,a.id aid,b.town btown,b.id bid from wwm2 a,wwm5 b where a.id=b.id)
   *  set atown=btown
 SQL> /
 
 rows updated.
 
这种方法的局限性就是需要PRIMARY 的支持.
 
方法四:
   1  declare
   2  cursor cur_wwm is select town,id from wwm5;
   3  begin
   4     for my_wwm in cur_wwm loop
   5     update wwm2 set town=my_wwm.town
   6     where id=my_wwm.id;
   7     end loop;
   8* end;
 SQL> /
 
PL/SQL procedure successfully completed.
 
SQL> select * from wwm2;
 
TOWN                         ID
 -------------------- ----------
 
 lllldf
 lllldf
 llll
 dddd
 lllldf
 lllldf
 lllldf
 lllldf
 ljjjjj
 lllldf                      
 
TOWN                         ID
 -------------------- ----------
 lllldf
 ljjjjj                      
 
这个方法是最灵活的了.
 
方法五:
 
注意,方法五只能适用于WWM5是WWM2的子集的时候.
      merge into wwm2
      using (select town,id from wwm5) b
      on (wwm2.id=b.id)
      when matched then update set town=b.town
   * when not matched then insert (town,id) values (null,null)
 SQL> /
 
 rows merged.
 
SQL> select * from wwm2;
 
TOWN                         ID
 -------------------- ----------
                                   ---注意这个地方,被插入了一个空值.因为WWM5的ID=9984在WWM2中不能匹配,根本原因是ORACLE9必须有WHEN NOT MATCHED子句,但是ORACLE10可以不许要,也就是ORACLE10可以不写WHEN NOT MATCHED ,就不必插入NULL值了,为解决这个问题,下一步会DELETE WWM5的ID=,这样一来就不会执行WHEN NOT MATCHED
 
 lllldf
 lllldf
 llll
 dddd
 lllldf
 lllldf
 lllldf
 lllldf
 ljjjjj                      
 
TOWN                         ID
 -------------------- ----------
 lllldf
 lllldf
 ljjjjj                      
 
 rows selected.
 
SQL> delete from wwm5 where id=;
 
 row deleted.
 
SQL>     merge into wwm2
 SQL>      using (select town,id from wwm5) b
 SQL>      on (wwm2.id=b.id)
 SQL>      when matched then update set town=b.town
 SQL>   * when not matched then insert (town,id) values (null,null)
 SQL> /
 
 rows merged.
 
       以上就是5种关连更新的例子了,希望能给开发人员解惑.
 
说明：如果select 子句可以返回多行记录，但返回适合where条件的记录只能是唯一的，否则将会报返回单行的select子句返回多行的错误，因为update只能跟据此处的where子句（内层where)进行相应记录的匹配更新，一次只能是一条。