Generate Sequential values for a column

Question

Hi,
I had a column in named 'ID' in the block of my form. I want to generate sequential values for the column without using sequence. Suppose if the table does not have any data it has display the starting value what i am providing.(ex. 101).If the data is there in the table If i am executing the query in the last null record it has to show the maxvalue+1 for that column. where can I write the code to get this logic? How to write the code? Can any body please solve my problem?
Thanks in advance
user1

Answer 1

Why don't you want to use a sequence? Do the ORDID values have to be sequential without any gaps in between?
If so, this requirement is quite hard to achieve in multi-user environment.
Some approaches:
Select the next available value from the database when creating a new record , e.g. in the WHEN-CREATE-RECORD-Trigger:
DECLARE
  CURSOR crMax IS
    SELECT NVL(MAX(ORDID), 0)+1
      FROM ORD;
BEGIN
  OPEN crMax;
  FETCh crMax INTO :ORD.ORDID;
  CLOSE crMax;
END;Pros: The user can see the new value when he starts entering the data.
Contras: When another user uses the same form to create a new record between the point of time user 1 starts entering the record and the point of time he saves, the same number will be taken again which will fail on insert for the second user (i assume there is a Unique Key on ORDID).
Result: This approach is not suitable in a multi-user environment.
Second approach:
Use the same logic as in approach 1, but select the next available number in the PRE-INSERT-trigger of the block:
Pros: The problem of approach 1 with two users getting the same number gets much more unlikely, for in general the COMMIT goes quite short after the PRE-INSERT has fired, so there will only be problems if both users save at the same moment.
Contras: Problem with duplicate numbers can still occur iunder special circumstances. The number is not shown until the Insert is issued against the database.
Result: This approach is possible, but you have to decide if the restrictions are bearable for your situation.
Further approaches:
Create a "Number table" either with just one record and one column which contains the next suitable number (lets say TAB_ORDID with column NEXT_ORDID) or with a number of records containing the next suitable numbers.
Then with the usage of SELECT FOR UPDATE you can lock a record, take the number from it and either update the row to the next value (one row apporach)or delete the row retrieved (multi row approach). Both cases require some more complex logic for retrieving the next number and can cause some trouble in multi-user-environments (ending up in all users who want to create records waiting for the one user who started and did not save correctly) if the locks are not handled correctly.