SQL TRIGGER：你如何限制某些东西被改变？

Question

在我的触发器中，如果列 Z 的值为 W，我必须限制列 X 更改为值 Y。IF 语句的外观如何？谢谢！

这是Class 表：

class         type  tara            nr_arme diametru_tun    deplasament
Ticonderoga   cr    Norvegia        16      23              4500
Maryland      vl    SUA             10      12              10000
Foxtrot       vl    Russia          20      45              5800
Buyan         cr    Russia          15      20              15000
Amphion       cr    Marea Britanie  13      14              12000

那些是ships：

name    class        anul_lansarii
Nava1   Ticonderoga  1995
Nava2   Amphion      1980
Nava3   Foxtrot      2005
Nava4   Buyan        1960
Nava5   Maryland     1970

这是Consequences 表。

name    batalie                 result
Nava1   Battle of Chemulpo Bay  scufundat
Nava3   Battle of Chemulpo Bay  avariat
Nava2   Battle of Hudson Bay    nevatamat
Nava4   Battle of Bengal Bay    nevatamat
Nava5   Battle of Bengal Bay    avariat

因此，如果船 (nava) 的类型为“cr”，则结果不能为“nevatamat”。它忽略更新。

Answer 1

测试用例：

SQL> create table class
  2    (class   varchar2(20),
  3     type    varchar2(2));

Table created.

SQL> create table ships
  2    (name    varchar2(5),
  3     class   varchar2(20));

Table created.

SQL> create table consequences
  2    (name    varchar2(5),
  3     result  varchar2(10));

Table created.

SQL> insert all
  2    into class values ('Ticonderoga' , 'cr')
  3    into class values ('Maryland'    , 'vl')
  4    --
  5    into ships values ('Nava1'       , 'Ticonderoga')
  6    into ships values ('Nava5'       , 'Maryland')
  7    --
  8    into consequences values ('Nava1', 'scufundat')
  9    into consequences values ('Nava5', 'avariat')
 10  select * from dual;

6 rows created.

SQL>

触发器：

SQL> create or replace trigger trg_biu_cons
  2    before insert or update on consequences
  3    for each row
  4  declare
  5    l_type class.type%type;
  6  begin
  7    if :new.result = 'nevatamat' then
  8       select c.type
  9         into l_type
 10         from class c
 11         where c.class = (select s.class
 12                          from ships s
 13                          where s.name = :new.name
 14                         );
 15       if l_type = 'cr' then
 16          raise_application_error(-20000, 'Ship''s type is CR; result can''t be NEVATAMAT');
 17       end if;
 18    end if;
 19  end;
 20  /

Trigger created.

SQL>

测试：

SQL> update consequences set result = 'nevatamat' where name = 'Nava1';
update consequences set result = 'nevatamat' where name = 'Nava1'
       *
ERROR at line 1:
ORA-20000: Ship's type is CR; result can't be NEVATAMAT
ORA-06512: at "SCOTT.TRG_BIU_CONS", line 13
ORA-04088: error during execution of trigger 'SCOTT.TRG_BIU_CONS'


SQL> update consequences set result = 'avariat'   where name = 'Nava1';

1 row updated.

SQL> update consequences set result = 'nevatamat' where name = 'Nava5';

1 row updated.

SQL>