android 数据库工具类MyDbHelper

package com.tdd.db.util;

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;

public class MyDbHelper {
    public static final String KEY_TITLE = "title";    //数据表字段
    public static final String KEY_BODY = "body";    //数据表字段
    public static final String KEY_ROWID = "_id";    //数据表字段

    private DatabaseHelper mDbHelper;    //SQLiteOpenHelper实例对象
    private SQLiteDatabase mDb;    //数据库实例对象
   
    //数据表创建语句
    private static final String DATABASE_CREATE  = "create table notes (_id integer primary key autoincrement, "
                    + "title text not null, body text not null);";

    private static final String DATABASE_NAME = "data";    //数据库名
    private static final String DATABASE_TABLE = "notes";    //数据库表名
    private static final int DATABASE_VERSION = 2;    //数据库版本号

    private final Context mCtx;    //上下文实例

    private static class DatabaseHelper extends SQLiteOpenHelper {    //数据库辅助类
        DatabaseHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }
        @Override
        public void onCreate(SQLiteDatabase db) {
            db.execSQL(DATABASE_CREATE);
        }
        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            db.execSQL("DROP TABLE IF EXISTS notes");
            onCreate(db);
        }
    }

    /**
     * Constructor - takes the context to allow the database to be opened/created
     *
     * @param ctx the Context within which to work
     */
    public MyDbHelper(Context ctx) {
        this.mCtx = ctx;
    }

    /**
     * Open the notes database. If it cannot be opened, try to create a new instance of the database.
* If it cannot be created, throw an exception to signal the failure
     *
     * @return this (self reference, allowing this to be chained in an initialization call)
     * @throws SQLException if the database could be neither opened or created
     */
    public MyDbHelper open() throws SQLException {
        mDbHelper = new DatabaseHelper(mCtx);
        mDb = mDbHelper.getWritableDatabase();
        return this;
    }
   
    public void close() {
        mDbHelper.close();
    }


    /**
     * Create a new note using the title and body provided. If the note is successfully created
* return the new rowId for that note, otherwise return a -1 to indicate failure.
     *
     * @param title the title of the note
     * @param body the body of the note
     * @return rowId or -1 if failed
     */
    public long createNote(String title, String body) {
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_TITLE, title);
        initialValues.put(KEY_BODY, body);
        return mDb.insert(DATABASE_TABLE, null, initialValues);
    }

    /**
     * Delete the note with the given rowId
     *
     * @param rowId id of note to delete
     * @return true if deleted, false otherwise
     */
    public boolean deleteNote(long rowId) {
        return mDb.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
    }

    /**
     * Return a Cursor over the list of all notes in the database
     *
     * @return Cursor over all notes
     */
    public Cursor fetchAllNotes() {
        return mDb.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_TITLE,KEY_BODY}, null, null, null, null, null);
    }

    /**
     * Return a Cursor positioned at the note that matches the given rowId
     *
     * @param rowId id of note to retrieve
     * @return Cursor positioned to matching note, if found
     * @throws SQLException if note could not be found/retrieved
     */
    public Cursor fetchNote(long rowId) throws SQLException {

        Cursor mCursor = mDb.query(true, DATABASE_TABLE, new String[] {KEY_ROWID,KEY_TITLE,
KEY_BODY}, KEY_ROWID + "=" + rowId, null, null, null, null, null);
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;
    }

    /**
     * Update the note using the details provided. The note to be updated is specified using the rowId,
*and it is altered to use the title and body values passed in
     *
     * @param rowId id of note to update
     * @param title value to set note title to
     * @param body value to set note body to
     * @return true if the note was successfully updated, false otherwise
     */
    public boolean updateNote(long rowId, String title, String body) {
        ContentValues args = new ContentValues();
        args.put(KEY_TITLE, title);
        args.put(KEY_BODY, body);
        return mDb.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
    }
}