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Oracle blocks reinserts

Consider a system in which there are several requests for participation in courses. We need a way to block duplication of registration in the system. I created the trigger as follows, but when I receive two requests from different connections at the same time (ms separately), they are both inserted. What am I doing wrong

create trigger enrollment_duplicates
before insert
on enrollment
for each row
begin
    select count(*) 
      into cnt 
      from enrollment 
     where user = :new.user 
       and course = :new.course 
       and status = 'Enrolled';
    if cnt > 0 then
        raise_application_error(-20001, 'User already enrolled in course');
    end if;
end;

EDIT:

This is easy enough if we make the user / course a unique constraint, but it is not. They may re-register depending on the condition.

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2 answers

. , Enrolled, ,

CREATE UNIQUE INDEX idx_stop_multiple_enrolls
    ON enrollment( (case when status = 'Enrolled' 
                         then user 
                         else null 
                      end),
                   (case when status = 'Enrolled' 
                         then course 
                         else null 
                      end) );

, Oracle , NULL, , status - Enrolled.

, USER ( USER), , - .

+6

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API (xapis). , , Table API (tapis), pl/sql. Xapis (ins/upd/del) , , , " ". . Ask Tom Xapi.

- xapi, , . ( ) pl/sql, :

create table enrollment
(
  id number,
  username varchar2(50),
  course varchar2(50),
  status varchar2(50),
  created_date date default sysdate not null
);

create index enrollment_idx
on enrollment(username, course)
logging
noparallel;

create or replace package enroll_pkg as
  err_already_enrolled      constant number := -20101;
  err_already_enrolled_msg  constant varchar2(50) := 'User is already enrolled';
  err_lock_request          constant number := -20102;
  err_lock_request_msg      constant varchar2(50) := 'Unable to obtain lock';
  enroll_lock_id            constant number := 42;

  function is_enrolled(i_username varchar2, i_course varchar2) return number;
  procedure enroll_user(i_username varchar2, i_course varchar2);

end;
/

create or replace package body enroll_pkg as

  -- returns 1=true, 0=false
  function is_enrolled(i_username varchar2, i_course varchar2) return number is
    l_cnt number := 0;
  begin

    -- run test if user is enrolled in this course
    select decode(count(1),0,0,1)
    into l_cnt
    from enrollment
    where username=i_username
    and course=i_course
    and status = 'ENROLLED';

    -- testing locks here
    --dbms_lock.sleep(5);

    return l_cnt;
  end;

  procedure enroll_user(i_username varchar2, i_course varchar2)
  is
    l_lock_result number;
    l_username enrollment.username%type;
    l_course enrollment.course%type;
  begin

    -- try to get lock (serialize access)  
    l_lock_result := dbms_lock.request(enroll_lock_id, dbms_lock.x_mode, 10, true);
    if (l_lock_result <> 0) then
      raise_application_error(err_lock_request,err_lock_request_msg || ' (' || l_lock_result || ')');
    end if;

    -- simple business rule: uppercase names & course
    l_username := upper(trim(i_username));
    l_course := upper(trim(i_course));

    if (is_enrolled(l_username, l_course) > 0) then
      raise_application_error(err_already_enrolled,err_already_enrolled_msg);
    end if;

    -- do other business logic checks, update other tables, logging, etc...

    -- add enrollment
    insert into enrollment(id,username,course,status) values
    (enroll_seq.nextval,l_username,l_course,'ENROLLED');

    commit;

    -- release lock
    l_lock_result := dbms_lock.release(enroll_lock_id);

  end;

end;
/

, :

exec enroll_pkg.enroll_user('Joe Smith','Biology');

, , , // xapi . , , .

+1

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