如何在Android Room数据库中将非空表列迁移为空

// Step 1: Create a new empty table for migration
@Entity(tableName = "new_table")
public class NewTableEntity {
    @PrimaryKey
    @ColumnInfo(name = "id")
    public int id;

    @ColumnInfo(name = "column1")
    public String column1;

    // Define other columns as needed
}

// Step 2: Create a migration class
public class MigrationFromOldToNew extends Migration {
    public MigrationFromOldToNew(int startVersion, int endVersion) {
        super(startVersion, endVersion);
    }

    @Override
    public void migrate(@NonNull SupportSQLiteDatabase database) {
        // Step 3: Query data from the old table
        Cursor cursor = database.query("SELECT * FROM old_table", null);

        // Step 4: Insert data into the new table
        if (cursor != null) {
            while (cursor.moveToNext()) {
                ContentValues values = new ContentValues();
                values.put("id", cursor.getInt(cursor.getColumnIndex("id")));
                values.put("column1", cursor.getString(cursor.getColumnIndex("column1")));
                // Insert other columns as needed
                database.insert("new_table", SQLiteDatabase.CONFLICT_REPLACE, values);
            }
            cursor.close();
        }

        // Step 5: Drop the old table
        database.execSQL("DROP TABLE IF EXISTS old_table");
    }
}

// Step 6: Update the database version and add migration to the database builder
@Database(entities = {NewTableEntity.class}, version = 2)
public abstract class AppDatabase extends RoomDatabase {
    public abstract NewTableDao newTableDao();

    public static AppDatabase buildDatabase(Context context) {
        return Room.databaseBuilder(context, AppDatabase.class, "app_database")
                .addMigrations(new MigrationFromOldToNew(1, 2))
                .build();
    }
}