How to represent and insert into an ordered list in SQL?
- by Travis
I want to represent the list "hi", "hello", "goodbye", "good day", "howdy" (with that order), in a SQL table:
pk | i | val
------------
1 | 0 | hi
0 | 2 | hello
2 | 3 | goodbye
3 | 4 | good day
5 | 6 | howdy
'pk' is the primary key column. Disregard its values.
'i' is the "index" that defines that order of the values in the 'val' column. It is only used to establish the order and the values are otherwise unimportant.
The problem I'm having is with inserting values into the list while maintaining the order. For example, if I want to insert "hey" and I want it to appear between "hello" and "goodbye", then I have to shift the 'i' values of "goodbye" and "good day" (but preferably not "howdy") to make room for the new entry.
So, is there a standard SQL pattern to do the shift operation, but only shift the elements that are necessary? (Note that a simple "UPDATE table SET i=i+1 WHERE i=3" doesn't work, because it violates the uniqueness constraint on 'i', and also it updates the "howdy" row unnecessarily.)
Or, is there a better way to represent the ordered list? I suppose you could make 'i' a floating point value and choose values between, but then you have to have a separate rebalancing operation when no such value exists.
Or, is there some standard algorithm for generating string values between arbitrary other strings, if I were to make 'i' a varchar?
Or should I just represent it as a linked list? I was avoiding that because I'd like to also be able to do a SELECT .. ORDER BY to get all the elements in order.
