Android 数据库

Android使用SQLite数据库系统。

为应用程序创建的数据库只能由其自身访问；其他应用程序无法访问它。

对于Android，在应用程序中以编程方式创建的SQLite数据库始终存储在 /data/data/package_name/databases 文件夹中。

升级数据库

要升级数据库，请将DATABASE_VERSION常量更改为高于上一个值的值。 例如，如果其上一个值为1，请将其更改为2:

package cn.w3cschool.myapplication3.app;
/* www.w3cschool.cn*/
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class DBAdapter {
    static final String KEY_ROWID = "_id";
    static final String KEY_NAME = "name";
    static final String KEY_EMAIL = "email";
    static final String TAG = "DBAdapter";

    static final String DATABASE_NAME = "MyDB";
    static final String DATABASE_TABLE = "contacts";
    static final int DATABASE_VERSION = 1;

    static final String DATABASE_CREATE =
            "create table contacts (_id integer primary key autoincrement, "
                    + "name text not null, email text not null);";

    final Context context;

    DatabaseHelper DBHelper;
    SQLiteDatabase db;

    public DBAdapter(Context ctx)
    {
        this.context = ctx;
        DBHelper = new DatabaseHelper(context);
    }

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

        @Override
        public void onCreate(SQLiteDatabase db)
        {
            try {
                db.execSQL(DATABASE_CREATE);
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
        {
            Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                    + newVersion + ", which will destroy all old data");
            db.execSQL("DROP TABLE IF EXISTS contacts");
            onCreate(db);
        }
    }
    //opens the database
    public DBAdapter open() throws SQLException
    {
        db = DBHelper.getWritableDatabase();
        return this;
    }

    //closes the database
    public void close()
    {
        DBHelper.close();
    }

    //insert a contact into the database
    public long insertContact(String name, String email)
    {
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_NAME, name);
        initialValues.put(KEY_EMAIL, email);
        return db.insert(DATABASE_TABLE, null, initialValues);
    }

    //deletes a particular contact
    public boolean deleteContact(long rowId)
    {
        return db.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
    }

    //retrieves all the contacts
    public Cursor getAllContacts()
    {
        return db.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_NAME,
                KEY_EMAIL}, null, null, null, null, null);
    }

    //retrieves a particular contact
    public Cursor getContact(long rowId) throws SQLException
    {
        Cursor mCursor =
                db.query(true, DATABASE_TABLE, new String[] {KEY_ROWID,
                                KEY_NAME, KEY_EMAIL}, KEY_ROWID + "=" + rowId, null,
                        null, null, null, null);
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;
    }

    //updates a contact
    public boolean updateContact(long rowId, String name, String email)
    {
        ContentValues args = new ContentValues();
        args.put(KEY_NAME, name);
        args.put(KEY_EMAIL, email);
        return db.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
    }
}