Android教程之SQlit数据库操作  

Android教程之SQlit数据库操作   

android.database.sqlite.SQLiteOpenHelperpublic       SQLiteOpenHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version)  Create a helper object to create, open, and/or manage a database. The database is not actually created or opened until one of getWritableDatabase() or getReadableDatabase() is called.
直到调用getWritableDatabase()函数或者getReadableDatabase()时才会创建或者打开数据库。（如果数据库没有创建，那么会先创建数据库）
Parameters 
context to use to open or create the database
name of the database file, or null for an in-memory database
factory to use for creating cursor objects, or null for the default
version number of the database (starting at 1); if the database is older, onUpgrade(SQLiteDatabase, int, int) will be used to upgrade the database

A helper class to manage database creation and version management.
SQLiteOpenHelper类是管理数据库生成和数据库版本建立的辅助类。
You create a subclass implementing onCreate(SQLiteDatabase), onUpgrade(SQLiteDatabase, int, int) and optionally onOpen(SQLiteDatabase), and this class takes care of opening the database if it exists, creating it if it does not, and upgrading it as necessary. Transactions are used to make sure the database is always in a sensible state

SQLiteOpenHelper，如果这个数据库存在，那么SQLiteOpenHelper负责管理数据库。
如果数据库不存在，那么SQLiteOpenHelper会建立一个数据库。
如果需要的话，升级数据库。

        private static final String DB_NAME = "CartDB.db";
        private static final int DB_VERSION = 2;
        private static final String TABLE_NAME_1 = "MyOrder";
        private static final String TABLE_NAME_2 = "OrderLine";

private static class DatabaseHelper extends SQLiteOpenHelper {
        DatabaseHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE " + TABLE_NAME_1 + " (" + "order_no"
        + " text not null, " + "type" + " text not null, " + "desc"
        + " text" + ");");
        db.execSQL("CREATE TABLE " + TABLE_NAME_2 + " (" + "order_no"
        + " text not null, " + "item_no" + " text not null, "
        + "QTY" + " text" + ");");
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        }
        }

 

   1.首先得新建数据库的代理类，通过这个类得到具体数据库的代理类。
mOpenHelper = new DatabaseHelper(this);
这个DatabaseHelper是继承SQLiteOpenHelper
SQLiteDatabase db = mOpenHelper.getWritableDatabase();
2.由于操作的是SQLiteDatabase数据，故得到一个SQLiteDatabase类，这个类里边的方法可以对数据库进行具体的操作。

建立数据库：
mOpenHelper = new DatabaseHelper(v.getContext());
        SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        String sql = "create table Student(" + "stud_no text not null, "
     &n