$ 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; --更新的條件表
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條數(shù)據(jù)就是正確的.
相信程序員是通過(guò)以下類(lèi)似的SQL更新的,這是錯(cuò)誤的,因?yàn)闆](méi)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條記錄被更新,符合條件的更新正確,不符合條件的也更新為NULL.以下是正確的方法
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: 與方法一道理相同,這里需要掌握EXIST的相關(guān)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.
這種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
這個(gè)方法是最靈活的了.
方法五:
注意,方法五只能適用于WWM5是WWM2的子集的時(shí)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
-------------------- ----------
---注意這個(gè)地方,被插入了一個(gè)空值.因?yàn)閃WM5的ID=9984在WWM2中不能匹配,根本原因是ORACLE9必須有WHEN NOT MATCHED子句,但是ORACLE10可以不許要,也就是ORACLE10可以不寫(xiě)WHEN NOT MATCHED ,就不必插入NULL值了,為解決這個(gè)問(wèn)題,下一步會(huì)DELETE WWM5的ID=9984,這樣一來(lái)就不會(huì)執(zhí)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種關(guān)連更新的例子了,希望能給開(kāi)發(fā)人員解惑.
說(shuō)明：如果select 子句可以返回多行記錄，但返回適合where條件的記錄只能是唯一的，否則將會(huì)報(bào)返回單行的select子句返回多行的錯(cuò)誤，因?yàn)閡pdate只能跟據(jù)此處的where子句（內(nèi)層where)進(jìn)行相應(yīng)記錄的匹配更新，一次只能是一條。