How to prevent duplication on a column with condition

Question

Hello everyone,
I need some advice here. At work, we have an Oracle APEX app that allow user to add new records with the automatic increment decision number based on year and group name.
Says if they add the first record , group name AA, for year 2012, they get decision number AA 1 2013 as their displayed record casein the report page.
The second record of AA in 2013 will be AA 2 2013.
If they add about 20 records , it will be AA 20 2013.
The first record for 2014 will be AA 1 2014.
However, recently , we get a user complaint about two records from the same group name have the same decision number.
When I looked into the history table, and find that the time gap between 2 record is just about 0.1 seconds.
Besides, we have lookup table that allows admin user to update the Start Sequence number with the restraint that it has to be larger than the max number of the current group name of the current year.
This Start sequence number and group name is stored together in a table.
And in some other special case,user can add a duplicate decision number for related record. (this is a new function)
The current procedure logic to add new record on the application are
_Get max(decision_number) from record table with chosen Group Name and current year.
_insert into the record table the new entered record with decision number + 1
_ update sequence number to the just added decision number.
So rather than utitlising APEX built-in automatic table modification process, I write a procedure that combine all the three process.
I run some for loop to continuously execute this procedure, and it seems it can autotically generate new unique decision number with time gap about 0.1 second.
However, when I increase the number of entry to 200, and let two users run 100 each.
If the time gap is about 0.01 second, Duplicate decision numbers appear.
What can I do to prevent the duplication ?
I cannot just apply a unique constraint here even for all three columns with condition, as it can have duplicate value in some special condition. I don't know much about using lock and its impact.
This is the content of my procedure
create or replace
PROCEDURE        add_new_case(
  --ID just use the trigger
  p_case_title IN varchar2,
  p_year IN varchar2,
  p_group_name IN VARCHAR2,
  --decisionnumber here
  p_case_file_number IN VARCHAR2,
  --active
  p_user IN VARCHAR2
AS
  default_value NUMBER;
    caseCount NUMBER;
  seqNumber NUMBER;
  previousDecisionNumber NUMBER;
BEGIN
  --execute immediate q'[alter session set nls_date_format='dd/mm/yyyy']';
  SELECT count(*)
        INTO caseCount
        FROM CASE_RECORD
        WHERE GROUP_ABBR = p_group_name
        AND to_number(to_char(create_date, 'yyyy')) = to_number(to_char(date_utils.get_current_date, 'yyyy'));
        SELECT max(decision_number)
        INTO previousDecisionNumber
        FROM CASE_RECORD
        WHERE GROUP_ABBR = p_group_name
        AND to_number(to_char(create_date, 'yyyy')) = to_number(to_char(date_utils.get_current_date, 'yyyy'));
        IF p_group_name IS NULL
        THEN seqNumber := 0;
        ELSE   
        SELECT seq_number INTO seqNumber FROM GROUP_LOOKUP WHERE ABBREVATION = p_group_name;
        END IF;
    IF caseCount > 0 THEN
           default_value := greatest(seqNumber, previousdecisionnumber)+1;
    ELSE
           default_value := 1;
    END IF; 
  INSERT INTO CASE_RECORD(case_title, decision_year, GROUP_ABBR, decision_number, case_file_number, active_yn, created_by, create_date)
  VALUES(p_case_title, p_year, p_group_name, default_value, p_case_file_number, 'Y', p_user, sysdate );
  --Need to update sequence here also
  UPDATE GROUP_LOOKUP
  SET SEQ_NUMBER = default_value
  WHERE ABBREVATION = p_group_name;
  COMMIT;
EXCEPTION
WHEN OTHERS THEN
    logger.error(p_message_text => SQLERRM
                ,p_message_code => SQLCODE
                ,p_stack_trace  => dbms_utility.format_error_backtrace
    RAISE;
END;
Many thanks in advance,
Ann

Answer 1

Why not using a sequence for populating the decision_number column ?
Sequence values are guaranteed to be unique so there's no need to lock anything.
You'll inevitably have gaps and no different groups will have the same decision_number in common.
Having to deal with consecutive numbers fixations you can proceed as
with
case_record as
(select 2012 decision_year,'AA' group_abbr,1 decision_number from dual union all
select 2012,'BB',2 from dual union all
select 2012,'AA',21 from dual union all
select 2012,'AA',22 from dual union all
select 2012,'BB',25 from dual union all
select 2013,'CC',33 from dual union all
select 2013,'CC',34 from dual union all
select 2013,'CC',36 from dual union all
select 2013,'BB',37 from dual union all
select 2013,'AA',38 from dual union all
select 2013,'AA',39 from dual union all
select 2013,'BB',41 from dual union all
select 2013,'AA',42 from dual union all
select 2013,'AA',43 from dual union all
select 2013,'BB',45 from dual
select decision_year,
       group_abbr,
       row_number() over (partition by decision_year,group_abbr order by decision_number) decision_number,
       decision_number sequence_number -- not shown (noone needs to know you're using a sequence)
  from case_record
order by decision_year,group_abbr,decision_number
DECISION_YEAR
GROUP_ABBR
DECISION_NUMBER
SEQUENCE_NUMBER
2012
AA
1
1
2012
AA
2
21
2012
AA
3
22
2012
BB
1
2
2012
BB
2
25
2013
AA
1
38
2013
AA
2
39
2013
AA
3
42
2013
AA
4
43
2013
BB
1
37
2013
BB
2
41
2013
BB
3
45
2013
CC
1
33
2013
CC
2
34
2013
CC
3
36
for retrieval (assuming decision_year,group_abbr,decision_number as being the key):
select decision_year,group_abbr,decision_number -- the rest of columns
  from (select decision_year,
               group_abbr,
-- the rest of columns
               row_number() over (partition by decision_year,group_abbr order by decision_number) decision_number
          from case_record
         where decision_year = :decision_year
           and group_abbr = :group_abbr
where decision_number = :decision_number
DECISION_YEAR
GROUP_ABBR
DECISION_NUMBER
2013
AA
4
if that's acceptable
Regards
Etbin