How to delete column in next version of room database Android?

Question

How to delete a column in next version of room database android.DROp or delete not working.?

@Database(entities = {User.class, AdTime.class}, version = 1, exportSchema = false)
  public abstract class AppDataBase extends RoomDatabase {

  private static AppDataBase INSTANCE;

  public abstract UserDao userModel();

  public abstract AdDao adModel();

  public static AppDataBase getInMemoryDatabase(Context context) {

    if (INSTANCE == null) {
        INSTANCE = Room.databaseBuilder(context, AppDataBase.class, "adrider")
                // To simplify the codelab, allow queries on the main thread.
                // Don't do this on a real app! See PersistenceBasicSample for an example.
                .allowMainThreadQueries()
              //  .addMigrations(MIGRATION_1_2)
                //  .fallbackToDestructiveMigration()
                .build();
      }
      return INSTANCE;
    }

  static final Migration MIGRATION_1_2=new Migration(1,2) {
      @Override
      public void migrate(@NonNull SupportSQLiteDatabase database) {
          database.execSQL("ALTER TABLE User "
                + "DROP Download");
      }
  };

}

Answer 1

You have to do 4 steps:
1. Create the new table
2. Copy the data
3. Remove the old table
4. Change the table name to the correct one

static final Migration MIGRATION_1_2 = new Migration(1, 2) {

    @Override
    public void migrate(SupportSQLiteDatabase database) {
        // Create the new table
        database.execSQL(
                "CREATE TABLE users_new (userid TEXT, username TEXT, last_update INTEGER, 
        PRIMARY KEY(userid))");
        // Copy the data
        database.execSQL(
                "INSERT INTO users_new (userid, username, last_update) SELECT userid, 
        username, last_update FROM users");
        // Remove the old table
        database.execSQL("DROP TABLE users");
        // Change the table name to the correct one
        database.execSQL("ALTER TABLE users_new RENAME TO users");
    }

};

Answer 2

My answer is very similar to what @Pedro Romao has given except that I have removed the extra unnecessary "NOT NULL" included in the sql statements. It should be added only where necessary as it might even cause an error where a column is allowed to be Null.

    // Create a table that would be your new table. Specify the type of each field and include NON NULL when field can't be null for example in the case of a primary key
    database.execSQL("CREATE TABLE Users_backup (id INTEGER NOT NULL, name TEXT, PRIMARY KEY(id))"); 
    // Copy the desired columns from the old table into this new table
    database.execSQL("INSERT INTO Users_backup (id, name) SELECT id, name FROM Users");
    // Delete the old table
    database.execSQL("DROP TABLE Users");
    // Rename the new table to the old table's name so that the rest of your code can recognize this table as the former one.
    database.execSQL("ALTER TABLE Users_backup RENAME TO Users");