Question

I have a database with a table, storing changes in account-balance across a couple of accounts, with three columns;

float   balance, #The account balance after the change Date    date,    #Date that balance change occurred int     aid      #Account that the balance change occurred on 

It contains a couple of entries for each day of the year, and I want to retrieve the balance of every five days. I also want it to separate between accounts (ie if two changes occurred on the same day, but on separate accounts, return both).

The problem is this: Sometimes there will be several days (or weeks) where there is no data available. When that occurs, I want to make sure to return the latest entry before the ‘hole’ in the dataset. This is a simplified version of the problem, the actual database is big (several gigabytes), the size is the reason why I want to return a subset of the data. It cannot use platform specific methods, because it needs to work on both oracle and mySQL.

My question is: Is there any way to do this fast? I would be able to write a query that gets the job done, but I am hoping there is some devil magic way of doing it that does not require lots of nested queries and aggregate functions..

Answer 1

I would use Andomar’s Period table idea, but I would try a slightly different final query. This assumes that your Account_Balances table has a PK on aid and date. If you ended up with two balances for the same account for the same exact date and time then you would get some duplicate rows.

SELECT      P.start_date,      P.end_date,      AB1.account_id,      AB1.balance FROM      Periods P LEFT OUTER JOIN Account_Balances AB1 ON      AB1.date <= P.end_date LEFT OUTER JOIN Account_Balances AB2 ON      AB2.aid = AB1.aid AND      AB2.date > AB1.date AND      AB2.date <= P.end_date WHERE      AB2.aid IS NULL 

If the account has no rows before or during the given period you will not get a row back for it.