Auto Increment column in database table

Question

Hello experts, I am using oracle 11g database at windows 7.I have to create a sequence object and a table with an auto increment ID column.For this I have created a trigger before insert to select next sequence from a sequence object. It works well but Now I have the current sequence is 7 and there are 6 records inserted in table. Now I delete two records.Thus I have only 5 records in table but my current sequence is still 7.So if I put it in id column then a new record will be inserted in table with 7 ID number after 5th ID number.I want this record should be inserted with 6 Id number. For this I tried to not use sequence object.I tried a pl/sql trigger before insert, which will count the all records in table and after increment i put it in ID column to insertion....Is this a professional way..? thank You regards aaditya

Answer 1

979801 wrote:
Hello experts, I am using oracle 11g database at windows 7.I have to create a sequence object and a table with an auto increment ID column.For this I have created a trigger before insert to select next sequence from a sequence object. It works well but Now I have the current sequence is 7 and there are 6 records inserted in table. Now I delete two records.Thus I have only 5 records in table but my current sequence is still 7.So if I put it in id column then a new record will be inserted in table with 7 ID number after 5th ID number.I want this record should be inserted with 6 Id number. For this I tried to not use sequence object.I tried a pl/sql trigger before insert, which will count the all records in table and after increment i put it in ID column to insertion....Is this a professional way..? thank You regards aaditya
Sequences only guarantee unique numbers.  You cannot (and should not) attempt to create gapless sesquences.  That's not how oracle works and will not scale to a multi user application.
Imagine two people try to insert records at the same time (and have yet to commit), the trigger you create will count the number of records and determine there are 5 records, so assign the next number of 6, for both people who are inserting records.  The first person to commit will get their data saved, and assuming you have a unique constriaint on that id, the second person will raise a duplicate key on insert or suchlike error.
Gapless sequential numbers are not appropriate to multi-user environments.  Such requirements are often given by managers or business people who do not understand the technology.
Think of it in terms of a real world office, but with people using a paper system instead of a computer.  The only way you can try to guarantee people get the next number and also re-use numbers that have been deleted is to have a single place where each person in the office goes to, to fetch the next number, and they have to queue up behind each other to get the next one off the list.  But if someone has removed an old record, they've got to wait in the same queue to go and put that number back in the pot for someone else to use.  It just doesn't work, even in a manual system.  Yes, people can guarantee that they're only getting unique numbers that nobody else is using, but they cannot guarantee that they are reusing and filling gaps etc.  It's an unrealistic expectation.