升级数据库的最佳写法

package com.example.databasetest;import android.content.Context;import android.database.sqlite.SQLiteDatabase;import android.database.sqlite.SQLiteOpenHelper;import android.widget.Toast;public class MyDatabaseHelper extends SQLiteOpenHelper {    public static final String CREATE_BOOK ="create table Book("            +" id integer primary key autoincrement , author text , "            +"price real, pages integer , name text" +            "category_id integer )";//用户直接安装第三版    public static  final String CREATE_CATEGORY = "create table Category ("            +"id integer primary key autoincrement,"            +"category_name text ,"            +"category_code integer)";    private Context mContext;    public   MyDatabaseHelper(Context context , String name , SQLiteDatabase.CursorFactory factory , int verson){//CursorFactory为查询数据时返回自定义的Cursor        super(context , name , factory , verson);        mContext = context ;    }    @Override    public void onCreate(SQLiteDatabase db) {        Toast.makeText(mContext,"Create succeed" ,Toast.LENGTH_SHORT).show();        db.execSQL(CREATE_BOOK);        db.execSQL(CREATE_CATEGORY);    }    @Override    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {//对数据库进行升级  执行的条件是在创建时的版本号大于上一次的版本号        switch (oldVersion){            case 1:db.execSQL(CREATE_CATEGORY);            case 2:db.execSQL("alter table Book add column category_id integer");            default:        }    }}

如果用户安装的是之前某一版的程序时，就会进入升级数据库的操作中。
注意每个case后无break保证用户可能从第一版直接升到第三版