android SQLite數據庫的使用

android提供了一個抽象類SQLiteOpenHelper用來管理數據庫，因此我們需要創建一個類來繼承它，並且重寫他的兩個方法onCreate()（創建數據庫）和onUpgrade()（更新數據庫）。他還有兩個重要的實例方法，這兩個方法都可一創建或打開一個數據庫，並且返回一個可讀寫的數據庫對象，區別是如果數據庫不可寫入時：

getReadableDatabase()  返回一個只可讀的

getWriteableDatabase()   出現異常

自定義MyDatabaseHelper

public class MyDatabaseHelper extends SQLiteOpenHelper{
	
	private Context mContext;
	
	public static final String CREATE_BOOK="create table Book("
			+ "id integer primary key autoincrement,"
			+ "author text,"
			+ "price real,"
			+ "pages integer,"
			+ "name text)";
	

	public MyDatabaseHelper(Context context, String name,
			CursorFactory factory, int version) {
		super(context, name, factory, version);
		mContext=context;
	}

	@Override
	public void onCreate(SQLiteDatabase db) {
		db.execSQL(CREATE_BOOK);
		Toast.makeText(mContext, "Create Succeeded", Toast.LENGTH_SHORT).show();
	}

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

}

對數據庫進行增刪查改(CRUD(Create,Retrieve,Update,Delete))操作，爲了給不熟悉sql語句的人方便操作，android提供了insert(),update(),delete(),query()方法進行操作數據庫。用法很簡單

public class MainActivity extends BaseActivity implements OnClickListener{
	
	private Button mCreateDbButton;
	
	private MyDatabaseHelper mHelper;
	
	private Button mAddDataButton;
	
	private Button mUpdateButton;
	
	private Button mDeleteButton;
	
	private Button mQueryButton;

	
	private static final String TAG="MainActivity";
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		//mHelper=new MyDatabaseHelper(this, "BookStore.db", null,1);
		mHelper=new MyDatabaseHelper(this, "BookStore.db",null,2);
	}

	@Override
	public void findId() {
		mCreateDbButton=(Button) findViewById(R.id.main_createDbButton);
		mAddDataButton=(Button) findViewById(R.id.main_addData_Button);
		mUpdateButton=(Button) findViewById(R.id.main_update_Button);
		mDeleteButton=(Button) findViewById(R.id.main_Delete_Button);
		mQueryButton=(Button) findViewById(R.id.main_Query_Button);
	}

	@Override
	public void setListener() {
		mCreateDbButton.setOnClickListener(this);
		mAddDataButton.setOnClickListener(this);
		mUpdateButton.setOnClickListener(this);
		mDeleteButton.setOnClickListener(this);
		mQueryButton.setOnClickListener(this);
	}

	@Override
	public void onClick(View v) {
		SQLiteDatabase db=mHelper.getWritableDatabase();
		ContentValues values=new ContentValues();
		switch (v.getId()) {
		case R.id.main_createDbButton:
			mHelper.getWritableDatabase();
			break;
		case R.id.main_addData_Button:
			//db=mHelper.getWritableDatabase();
			//values=new ContentValues();
			values.put("name", "c語言從入門到放棄");
			values.put("author", "c語言");
			values.put("pages", 520);
			values.put("price", 50.2);
			db.insert("Book", null, values);
			values.clear();
			
			values.put("name", "java從入門到放棄");
			values.put("author", "java");
			values.put("pages", 385);
			values.put("price", 47.8);
			db.insert("Book", null, values);
			values.clear();
			break;
		case R.id.main_update_Button:
			values.put("price", 88.8);
			db.update("Book", values, "name=?",new String[]{"c語言從入門到放棄"});
			values.clear();
			break;
		case R.id.main_Delete_Button:
			db.delete("Book", "pages>?",new String[]{"500"});
			break;
		case R.id.main_Query_Button:
			Cursor cursor=db.query("Book", null, null, null, null, null, null);
			if(cursor.moveToFirst()){
				do{
					String name=cursor.getString(cursor.getColumnIndex("name"));
					String author=cursor.getString(cursor.getColumnIndex("author"));
					int pages=cursor.getInt(cursor.getColumnIndex("pages"));
					double price=cursor.getDouble(cursor.getColumnIndex("price"));
					Log.i(TAG,name+author+pages+price);
					
				}while(cursor.moveToNext());
			}
			break;
		default:
			break;
		}
	}
}

也可以使用sql語句進行操作，有時候用SQL語句操作更簡單。

增刪查改

db.execSQL("insert into Book(name,author,pages,price)values(?,?,?,?)",
					new String[]{"c#語言","c#","452","12.9"});

db.execSQL("delete from Book where price<?",new String[]{"50"});

Cursor cursor=db.rawQuery("select * from Book where name=?",new String[]{"c#語言"});			

db.execSQL("update Book set price=? where name=?",new String[]{"100","c#語言"});

升級數據庫的最佳寫法,我在第二個版本里面加了一個表Category，第三個版本里面在Book表中加了一個字段category_id

如果是新用戶直接安裝就執行oncreate()方法，兩個表直接創建,如果是第一個版本的用戶升級到第三個版本，就執行onUpgrade()方法，先創建第二個表，再在Book表中添加字段的操作。

注意switch case語句不寫break

public class MyDatabaseHelper extends SQLiteOpenHelper{
	
	private Context mContext;
	
	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)";
	
	public static final String CREATE_CATEGORY="create table Category("
			+ "id integer primary key autoincrement,"
			+ "category_name text,"
			+ "category_code integer)";

	public MyDatabaseHelper(Context context, String name,
			CursorFactory factory, int version) {
		super(context, name, factory, version);
		mContext=context;
	}

	@Override
	public void onCreate(SQLiteDatabase db) {
		db.execSQL(CREATE_BOOK);
		db.execSQL(CREATE_CATEGORY);
		Toast.makeText(mContext, "Create Succeeded", Toast.LENGTH_SHORT).show();
	}

	@Override
	public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
		switch (oldVersion) {
		case 1:
			db.execSQL(CREATE_CATEGORY);
		case 2:
			db.execSQL("alter table Book add column category_id integer");
		default:
			
		}
	}

}