How can data be migrated between database versions or structures?

To transfer data between database versions or change its structure in Android, database migrations are usually used. In SQLite, which is often used in Android, migrations are implemented by changing the schema and transferring data.

Main approaches:

1. Using SQLiteOpenHelper and the onUpgrade() method:

   • When the database version changes (the version parameter in the SQLiteOpenHelper constructor), onUpgrade() is called.
   • In this method, you can execute SQL queries to modify the structure (ALTER TABLE, CREATE TABLE, etc.) and transfer data.

2. Room Persistence Library:

   • Room supports migrations through the Migration class, where you describe SQL queries for transitioning from one version to another.
   • Example migration:

static final Migration MIGRATION_1_2 = new Migration(1, 2) {
    @Override
    public void migrate(SupportSQLiteDatabase database) {
        database.execSQL("ALTER TABLE users ADD COLUMN last_update INTEGER");
    }
};

3. Export and import data:

   • In some cases, data is exported to a file (e.g., JSON or CSV), then a new database structure is created, and data is imported back.

4. Using third-party migration libraries:

   • There are libraries that simplify migrations and version management of the database.

Thus, transferring data between database versions in Android is a migration process that involves changing the structure and saving/transferring data using built-in mechanisms of SQLite or Room.