how can I selectively update some SQLite table to preserve user data for an iphone app update?

can you selectively update certain sqllite tables when updating an iphone app, in order to preserve user stored data? ho开发者_StackOverflow中文版w? appreciate the help!

At the simplest level, you'll need to:

1. Store some version number information in the SQL database in the app's document directory.

2. When your app launches, you can compare this version data to the copy in your bundle.

3. If the version is different, you'll then need to activate a "updater" class, the responsibility of which is to:

   3.1. Check for the existence of each table.

   3.2. If it exists, load any existing data into a suitable data structure (an NSDictionary most likely), cull the table and create it in the "current" format, providing sensible defaults where no data exists.

As you can imagine, in the above scenario the updater class effectively needs to know how to create each table in turn, which isn't ideal - an alternative approach being to store a list of ALTER TABLE statements for each version and then apply them in turn until the database structure is up to date.