10. 某个App的开发应用过程中，在版本1时，建立了一个学生表Student（sno integer， name varchar（20）， age integer）；在版本2时需要对该表增加一个字段手机号码phoneNumber;在版本3时，需要增加一个课程表course（id integer, name varchar, credit integer）,根据此种情况完成下面的代码： （1）版本2时的SQLiteOpenHelper类的实现； （2）编写一个方法，查询学生表中的所有数据； （3）版本3时的SQLiteOpenHelper类的实现。

（1）版本2时的SQLiteOpenHelper类的实现：

public class MyDatabaseHelper extends SQLiteOpenHelper {
    private static final String DATABASE_NAME = "myapp.db";
    private static final int DATABASE_VERSION = 2;

    public MyDatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // create student table in version 1
        db.execSQL("CREATE TABLE Student (sno INTEGER, name VARCHAR(20), age INTEGER);");
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        if (oldVersion < 2) {
            // add phoneNumber column to student table in version 2
            db.execSQL("ALTER TABLE Student ADD COLUMN phoneNumber VARCHAR(20);");
        }
    }
}

（2）编写一个方法，查询学生表中的所有数据：

public List<Student> getAllStudents() {
    List<Student> studentList = new ArrayList<>();
    SQLiteDatabase db = getReadableDatabase();
    Cursor cursor = db.query("Student", null, null, null, null, null, null);
    if (cursor != null) {
        while (cursor.moveToNext()) {
            Student student = new Student();
            student.setSno(cursor.getInt(cursor.getColumnIndex("sno")));
            student.setName(cursor.getString(cursor.getColumnIndex("name")));
            student.setAge(cursor.getInt(cursor.getColumnIndex("age")));
            student.setPhoneNumber(cursor.getString(cursor.getColumnIndex("phoneNumber")));
            studentList.add(student);
        }
        cursor.close();
    }
    db.close();
    return studentList;
}

（3）版本3时的SQLiteOpenHelper类的实现：

public class MyDatabaseHelper extends SQLiteOpenHelper {
    private static final String DATABASE_NAME = "myapp.db";
    private static final int DATABASE_VERSION = 3;

    public MyDatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // create student table in version 1
        db.execSQL("CREATE TABLE Student (sno INTEGER, name VARCHAR(20), age INTEGER, phoneNumber VARCHAR(20));");
        // create course table in version 3
        db.execSQL("CREATE TABLE Course (id INTEGER, name VARCHAR(20), credit INTEGER);");
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        if (oldVersion < 2) {
            // add phoneNumber column to student table in version 2
            db.execSQL("ALTER TABLE Student ADD COLUMN phoneNumber VARCHAR(20);");
        }
        if (oldVersion < 3) {
            // create course table in version 3
            db.execSQL("CREATE TABLE Course (id INTEGER, name VARCHAR(20), credit INTEGER);");
        }
    }
}