Condition in trigger

Question

Dear All,
I wrote a trigger and i used conditions, but it doesn´t work.
i use oracle 11g .
would you please recorrect it?
thanks, if u need i can provide trable information but i just want to correct it syntactically.tnx.
i want to check, if the population of capital in a country is a city with the most population , then it sends use the first error message, other wise it means that there are some cities with population more that the capital city and we give another error message.
{code}
create or replace
trigger capital city
before update or insert or delete on country
for each row
when exists ((select population from city ci where ci.name=capital)<(select max(population) from city ci2 where ci2.country=code))
begin
raise_application_error(-20000,'verletzung der integrität');
end;
when ((select population from city ci where ci.name=capital)=(select max(population) from city ci2 where ci2.country=code))
begin
raise_application_error(-20000,'die Hauptstadt sind die größten städte der jeweiligen länder');
end;
{code}
or i also wrote this one, but both of them don´t work.
{code}
create or replace trigger bigcapital
after insert or update of population on city
for each row
when exists( select count(*) from country co where co.capital<> (select ci.name from city ci where ci.country=co.code and ci.population= (select max(population)from city ci1 where ci1.country= ci.country) ) )
begin
raise_application_error(-200002,'Verletzung der Intergrität');
end;
{code}
Message was edited by: 1006212

Answer 1

So your requirement is that in your CITY table the population of a city should be always less than that of the capital city for a given country?
The best way would be to implement the constraint using Transaction-API build using PL/SQL.
But any way here is another approach using materialized view.
SQL> create table city
  2  (
  3    name        varchar2(100),
  4    capital     varchar2(1),
  5    country     varchar2(100),
  6    population  integer
  7  );
Table created.
SQL> create materialized view log on city
  2  with rowid, sequence(country, capital, population)
  3  including new values;
Materialized view log created.
SQL> create materialized view city_mv refresh fast on commit with primary key
  2  as
  3  select country
  4       , max(case when capital = 'Y' then population end) capital_population
  5       , max(case when capital = 'N' then population end) other_population
  6    from city
  7   group
  8      by country;
Materialized view created.
SQL> alter materialized view city_mv add constraint city_mv_check_1
  2  check (other_population < capital_population);
Materialized view altered.
SQL> insert into city (name, capital, country, population)
  2  values ('DELHI', 'Y', 'INDIA', 100000);
1 row created.
SQL> commit;
Commit complete.
SQL> insert into city (name, capital, country, population)
  2  values ('MUMBAI', 'N', 'INDIA', 1100000);
1 row created.
SQL> commit;
commit
ERROR at line 1:
ORA-12008: error in materialized view refresh path
ORA-02290: check constraint (ARBORU.CITY_MV_CHECK_1) violated
SQL> insert into city (name, capital, country, population)
  2  values ('MUMBAI', 'N', 'INDIA', 10000);
1 row created.
SQL> commit;
Commit complete.