postgresql trigger on insert not bigger than limit -

now have table courseenroll table has student_id , section_id, , section table has different section_id , limit number of students. , student table has difference students_id.

i beginner trigger. need make trigger constrains. such student insert 1 one web user interface jsp if number of students bigger limit of students in section, should display error message user. "the error message displayed should include error text coming db server, possibly accompanied own message identifies constraint violated. "

i try follow example link sql using trigger constraint modify below:

create trigger enroll_out_of_limit on courseenroll after insert, update if exists (            select *               section t                   inner join (                               select section_id, count(*) limitstu                                 courseenroll                                group                                    section_id                                    ) e                                   on t.section_id = e.section_id                                      , e.limitstu > t.limitstu           ) begin raiserror ('a teachers''s student tally high accept new students.', 16, 1); rollback transaction; return  end; 

while give me error @ beginning of code around "on" ,so how fix this? thanks

the section table has 2 attributes: section_id , limitstudent

the student table has 1 attributes: student_id

the courseenroll has 2 attributes: student_id , section_id

as long number of students in 1 section courseenroll table not bigger corresponding limit student section fine.

i followed tutorial have following trigger while gave me error around select statement

    create or replace function enroll_out_of_limit()   returns trigger $body$ begin     if (select *          section t                 inner join (                               select section_id, count(*) limitstu                               courseenroll                               group section_id                                    ) e                                   on t.section_id = e.section_id                                      , e.limitstu > t.limitstu )                                      raise exception 'this section enrolllist full!';           end if;  end; $body$   language plpgsql volatile   cost 100; alter function enroll_out_of_limit()   owner postgres; 

if change sql below, can create trigger when try insert, gave error "the query has no goal"

    create or replace function enroll_out_of_limit()  returns trigger  $body$ begin          select t.limitstu, e.currentsum         section t         inner join (                 select section_id, count(*) currentsum                 courseenroll                 group section_id                       ) e         on t.section_id = e.section_id ;         if e.currentsum = t.limitstu                     raise exception 'this section enrolllist full!';         end if;   end; $body$     language 'plpgsql';  create trigger enroll_out_of_limit before insert on courseenroll each row execute procedure enroll_out_of_limit();