正如大多数其他人所指出的：由于您的数据模型，您遇到了一个大问题。为这个模型编写的大多数代码将比它需要的要困难得多。我已经通过上下投票以及在某些 cmets 中说过，但还不够。
如果您继续前进，那么下面的代码将演示需要做什么。希望它吓到你:-)
示例表：
SQL> create table vehicles (id,registrationnumber,lastallocationusername,lastallocationdate,lastallocationid)
2 as
3 select 1, 1, 'Me', sysdate-1, 2 from dual union all
4 select 2, 2, 'Me', sysdate, 3 from dual
5 /
Table created.
SQL> create table allocations (id,vehicleid,username,mydate)
2 as
3 select 1, 1, 'Me', sysdate-2 from dual union all
4 select 2, 1, 'Me', sysdate-1 from dual union all
5 select 3, 2, 'Me', sysdate-1 from dual
6 /
Table created.
触发器必须查看它自己的表以确定最后一次分配。 Oracle 通过引发变异表错误来防止这种类型的脏读。为了避免这种情况，我创建了一个 SQL 类型和一个包：
SQL> create type t_vehicle_ids is table of number;
2 /
Type created.
SQL> create package allocations_mutating_table
2 as
3 procedure reset_vehicleids;
4 procedure store_vehicleid (p_vehicle_id in vehicles.id%type);
5 procedure adjust_vehicle_last_allocation;
6 end allocations_mutating_table;
7 /
Package created.
SQL> create package body allocations_mutating_table
2 as
3 g_vehicle_ids t_vehicle_ids := t_vehicle_ids()
4 ;
5 procedure reset_vehicleids
6 is
7 begin
8 g_vehicle_ids.delete;
9 end reset_vehicleids
10 ;
11 procedure store_vehicleid (p_vehicle_id in vehicles.id%type)
12 is
13 begin
14 g_vehicle_ids.extend;
15 g_vehicle_ids(g_vehicle_ids.count) := p_vehicle_id;
16 end store_vehicleid
17 ;
18 procedure adjust_vehicle_last_allocation
19 is
20 begin
21 update vehicles v
22 set ( v.lastallocationusername
23 , v.lastallocationdate
24 , v.lastallocationid
25 ) =
26 ( select max(a.username) keep (dense_rank last order by a.mydate)
27 , max(a.mydate)
28 , max(a.id) keep (dense_rank last order by a.mydate)
29 from allocations a
30 where a.vehicleid = v.id
31 )
32 where v.id in (select column_value from table(cast(g_vehicle_ids as t_vehicle_ids)))
33 ;
34 end adjust_vehicle_last_allocation
35 ;
36 end allocations_mutating_table;
37 /
Package body created.
然后3个数据库触发器将更新代码从行级别移动到语句级别，从而规避了变异表错误：
SQL> create trigger allocations_bsiud
2 before insert or update or delete on allocations
3 begin
4 allocations_mutating_table.reset_vehicleids;
5 end;
6 /
Trigger created.
SQL> create trigger allocations_ariud
2 after insert or update or delete on allocations
3 for each row
4 begin
5 allocations_mutating_table.store_vehicleid(nvl(:new.vehicleid,:old.vehicleid));
6 end;
7 /
Trigger created.
SQL> create trigger allocations_asiud
2 after insert or update or delete on allocations
3 begin
4 allocations_mutating_table.adjust_vehicle_last_allocation;
5 end;
6 /
Trigger created.
还有一个小测试来验证它是否可以在单用户环境中工作：
SQL> select * from vehicles
2 /
ID REGISTRATIONNUMBER LA LASTALLOCATIONDATE LASTALLOCATIONID
---------- ------------------ -- ------------------- ----------------
1 1 Me 13-05-2010 14:03:43 2
2 2 Me 14-05-2010 14:03:43 3
2 rows selected.
SQL> insert into allocations values (4, 1, 'Me', sysdate)
2 /
1 row created.
SQL> select * from vehicles
2 /
ID REGISTRATIONNUMBER LA LASTALLOCATIONDATE LASTALLOCATIONID
---------- ------------------ -- ------------------- ----------------
1 1 Me 14-05-2010 14:03:43 4
2 2 Me 14-05-2010 14:03:43 3
2 rows selected.
SQL> update allocations
2 set mydate = mydate - 2
3 where id = 4
4 /
1 row updated.
SQL> select * from vehicles
2 /
ID REGISTRATIONNUMBER LA LASTALLOCATIONDATE LASTALLOCATIONID
---------- ------------------ -- ------------------- ----------------
1 1 Me 13-05-2010 14:03:43 2
2 2 Me 14-05-2010 14:03:43 3
2 rows selected.
SQL> delete allocations
2 where id in (2,4)
3 /
2 rows deleted.
SQL> select * from vehicles
2 /
ID REGISTRATIONNUMBER LA LASTALLOCATIONDATE LASTALLOCATIONID
---------- ------------------ -- ------------------- ----------------
1 1 Me 12-05-2010 14:03:43 1
2 2 Me 14-05-2010 14:03:43 3
2 rows selected.
现在您所要做的就是添加一些序列化以使其在多用户环境中 100% 工作。但希望这个例子足够可怕。
问候，
抢。