How do i resolve this trigger in pl/sql?

I want to have a trigger that no more than 3 employees work on a given project this is the code that i have but doesn’t work very well, sometimes work and sometimes doesn’t work.

create or replace trigger t_maxim_empleats_projecte
before insert or update of codi_proj
on empleats
for each row
declare
contador number(5);
begin
select count(codi_proj) into contador from empleats where codi_proj= :new.codi_proj;
if contador > 3 then
RAISE_APPLICATION_ERROR(-20000, 'No poden haver-hi més de 3 empleats en un
mateix projecte.');
end if;
end;

This is all i have in my SQL tables:

create table Projectes(
    codi_proj   number(5), 
    nom_proj    varchar2(25),
    pressupost  number(10,2),
    primary key (codi_proj)
);

create table Empleats(
    codi_emp    number(5),
    nom_emp varchar2(15),
    sou     number(10,2),
    codi_dept   number(5),
    codi_proj   number(5),
    data_alta   date,
    primary key (codi_emp),
        foreign key (codi_dept)   references Departaments(codi_dept) on delete set null,
        foreign key (codi_proj)   references Projectes(codi_proj) on delete set null
);

insert into projectes(codi_proj, nom_proj, pressupost)
values (1, 'Daisy', 240000);

insert into projectes(codi_proj, nom_proj, pressupost)
values (2, 'CLAM', 63000);

insert into projectes(codi_proj, nom_proj, pressupost)
values (3, 'Vocal Processor', 600000);

insert into empleats(codi_emp, nom_emp, sou, codi_dept, codi_proj,data_alta)
values (1, 'Maria', 21000, 1, 1,TO_DATE('10/10/1980','dd/mm/yyyy'));

insert into empleats(codi_emp, nom_emp, sou, codi_dept, codi_proj,data_alta)
values (2, 'Josep', 18000, 1, 1,TO_DATE('01/08/1982','dd/mm/yyyy'));

insert into empleats(codi_emp, nom_emp, sou, codi_dept, codi_proj,data_alta)
values (3, 'Ramon', 48000, 4, 2,TO_DATE('05/04/2005','dd/mm/yyyy'));

Answer

You are not going to do what you want at least with the current row level trigger approach. The reason being that a row level trigger can not access the table that caused it to fire. As a result the statement “select … from empleats … will throw the exception “ORA-04091: table is mutating, trigger/function may not see it” because the trigger fires in response to DML against table empleats. A better process is to make this check in a Business Rules layer, or even the application level. However, if you insist on a trigger, it can be accomplished via an after statement trigger. That trigger does have to (or at least should) deal with possibility multiple projects exceeding the max employee limit. So (See fiddle):

create or replace trigger limit_3_emp_per_proj_ais
           after insert on employees 
declare
   k_new_msg_line    constant            varchar2(3)  := chr(10) || '  ';
   k_max_emp_message constant            varchar2(80) := 
       'No rows inserted!' || k_new_msg_line || 'Following Projects have exceed max of 3 employees:'; 
   cursor proj_over_3_emp is
         select proj.name, proj.proj_id, count(*)
           from projects  proj 
           join employees emp
             on (emp.proj_id = proj.proj_id)
          group by proj.proj_id, proj.name    
         having count(*) > 3 
          order by proj.proj_id;

   l_proj_exceeds_3_emp                varchar2(3500) := null;           
begin 
   for proj_emp in proj_over_3_emp  
   loop 
       l_proj_exceeds_3_emp := l_proj_exceeds_3_emp || k_new_msg_line ||
                               proj_emp.name || '(' || proj_emp.proj_id || ')';
   end loop; 

   if l_proj_exceeds_3_emp is not null
   then 
      raise_application_error( -20199,k_max_emp_message || l_proj_exceeds_3_emp);
   end if;

end limit_3_emp_per_proj_ais;