[转]oracle update set select from 关联更新

$ sqlplus user/pass 

SQL*Plus: Release 9.2.0.6.0 - Production on Wed Aug 2 17:38:39 2006 

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved. 


 Connected to:
 Oracle9i Enterprise Edition Release 9.2.0.6.0 - 64bit Production
 With the Partitioning, OLAP and Oracle Data Mining options
 JServer Release 9.2.0.6.0 - Production 

SQL> select * from wwm2;        --要更新的表 

TOWN                         ID
 -------------------- ----------
 222                         222
 111                         111
 ww'jj                       111
 llll                       1111
 dddd                       2222
 lllldf                      111
 lllldf                      111
 dsafdf                      111
 3435                        111
 ljjjjj                      222
 dsafdf                      111

TOWN                         ID
 -------------------- ----------
 3435                        111
 ljjjjj                      222


 SQL> select * from wwm5;            --更新的条件表

TOWN                         ID
 -------------------- ----------
 lllldf                      111
 test                       9984

SQL> select wwm2.* from wwm2,wwm5 where wwm2.id=wwm5.id
   2  /

TOWN                         ID
 -------------------- ----------
 111                         111
 ww'jj                       111
 lllldf                      111
 lllldf                      111
 dsafdf                      111
 3435                        111
 dsafdf                      111
 3435                        111

8 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)
   2  /

13 rows updated.

SQL> select * from wwm2;

TOWN                         ID
 -------------------- ----------
                             222
 lllldf                      111
 lllldf                      111
                            1111
                            2222
 lllldf                      111
 lllldf                      111
 lllldf                      111
 lllldf                      111
                             222
 lllldf                      111

TOWN                         ID
 -------------------- ----------
 lllldf                      111
                             222
 13 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
   2  set town=(select town from wwm5 where wwm5.id=wwm2.id)
   3  where id=(select wwm5.id from wwm5 where wwm5.id=wwm2.id)
   4  /

8 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 1 from wwm5 where wwm5.id=wwm2.id)
 8 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)
   2  set atown=btown
   3  /
 set atown=btown
     *
 ERROR at line 2:
 ORA-01779: cannot modify a column which maps to a non key-preserved table

  1* alter table wwm5 add primary key (id)
 SQL> /

Table altered.

  1  update (select a.town atown,a.id aid,b.town btown,b.id bid from wwm2 a,wwm5 b where a.id=b.id)
   2*  set atown=btown
 SQL> /

8 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
 -------------------- ----------
 222                         222
 lllldf                      111
 lllldf                      111
 llll                       1111
 dddd                       2222
 lllldf                      111
 lllldf                      111
 lllldf                      111
 lllldf                      111
 ljjjjj                      222
 lllldf                      111

TOWN                         ID
 -------------------- ----------
 lllldf                      111
 ljjjjj                      222

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.
   1   merge into wwm2
   2   using (select town,id from wwm5) b
   3   on (wwm2.id=b.id)
   4   when matched then update set town=b.town
   5* when not matched then insert (town,id) values (null,null)
 SQL> /

9 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=9984,这样一来就不会执行WHEN NOT MATCHED
 222                         222
 lllldf                      111
 lllldf                      111
 llll                       1111
 dddd                       2222
 lllldf                      111
 lllldf                      111
 lllldf                      111
 lllldf                      111
 ljjjjj                      222

TOWN                         ID
 -------------------- ----------
 lllldf                      111
 lllldf                      111
 ljjjjj                      222

14 rows selected.

SQL> delete from wwm5 where id=9984;

1 row deleted.

SQL>  1   merge into wwm2                             
 SQL>   2   using (select town,id from wwm5) b
 SQL>   3   on (wwm2.id=b.id)
 SQL>   4   when matched then update set town=b.town
 SQL>   5* when not matched then insert (town,id) values (null,null)
 SQL> /

8 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。