安卓开发:数据库操作(详细)
程序员文章站
2022-06-23 20:42:03
安卓使用的数据库是sqlite 创建方式:新建一个类继承SQLiteOpenHelper 这里第二个参数:数据库的名字,第三个参数为结果集(游标)写成null即可,最后一个参数为数据库的版本,写1即可 在MainActivity中新建实例即可创建数据库: 好的,运行即可创建数据库成功 接下来看下My ......
安卓使用的数据库是sqlite
创建方式:新建一个类继承SQLiteOpenHelper
package org.dreamtech.sqlite; import android.content.Context; import android.database.sqlite.SQLiteDatabase; import android.database.sqlite.SQLiteOpenHelper; public class MyOpenHelper extends SQLiteOpenHelper { public MyOpenHelper(Context context) { super(context, "dreamtech.db", null, 1); } @Override public void onCreate(SQLiteDatabase db) { } @Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { } }
这里第二个参数:数据库的名字,第三个参数为结果集(游标)写成null即可,最后一个参数为数据库的版本,写1即可
在MainActivity中新建实例即可创建数据库:
package org.dreamtech.sqlite; import android.os.Bundle; import android.app.Activity; import android.database.sqlite.SQLiteDatabase; public class MainActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); MyOpenHelper myOpenHelper = new MyOpenHelper(getApplicationContext()); // 获取一个可写的数据库,如果数据库不存在,则新建数据库 SQLiteDatabase sqLiteDatabase = myOpenHelper.getWritableDatabase(); } }
好的,运行即可创建数据库成功
接下来看下MyOpenHelper中的方法:
oncreate:数据库第一次创建时候执行的方法
适合于初始化表结构
示例:
@Override public void onCreate(SQLiteDatabase db) { db.execSQL("create table info(_id integer primary key autoincrement,name varchar(20))"); }
另一个方法:onupgrade方法:
当数据库版本升级的时候调用(上边设置为1版本,如果升级成2,会运行这里的方法)
示例:这里可以修改表结构
@Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { db.execSQL("alter table info add tel varchar(20)"); }
注意:版本只能升,不能降级
接下来做一个增删改查的小案例:
使用原始方法:
package org.dreamtech.sqlite; import android.os.Bundle; import android.view.View; import android.app.Activity; import android.database.Cursor; import android.database.sqlite.SQLiteDatabase; public class MainActivity extends Activity { private MyOpenHelper myOpenHelper; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); myOpenHelper = new MyOpenHelper(getApplicationContext()); } // 增 public void click1(View v) { SQLiteDatabase db = myOpenHelper.getWritableDatabase(); db.execSQL("insert into info(name,tel) values(?,?)", new Object[] { "zhangsan", "110" }); db.close(); } // 删 public void click2(View v) { SQLiteDatabase db = myOpenHelper.getWritableDatabase(); db.execSQL("delete from info where name=?", new Object[] { "zhangsan" }); db.close(); } // 改 public void click3(View v) { SQLiteDatabase db = myOpenHelper.getWritableDatabase(); db.execSQL("update info set tel=? where name=?", new Object[] { "120", "zhangsan" }); db.close(); } // 查 public void click4(View v) { SQLiteDatabase db = myOpenHelper.getWritableDatabase(); Cursor cursor = db.rawQuery("select * from info", null); if (cursor != null && cursor.getCount() > 0) { while (cursor.moveToNext()) { String name = cursor.getString(1); String tel = cursor.getString(2); System.out.println(name + " " + tel); } } db.close(); } }
确实成功了,但是存在问题:sql语句只要一处出错,将难以发现,浪费大量时间
唯一的优点:多表查询
通常操作数据库是采用谷歌封装好的API:
优点:不易写错,有返回值,方便开发
缺点:涉及多表操作不便
package org.dreamtech.sqlite; import android.os.Bundle; import android.view.View; import android.widget.Toast; import android.app.Activity; import android.content.ContentValues; import android.database.Cursor; import android.database.sqlite.SQLiteDatabase; public class MainActivity extends Activity { private MyOpenHelper myOpenHelper; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); myOpenHelper = new MyOpenHelper(getApplicationContext()); } // 增 public void click1(View v) { SQLiteDatabase db = myOpenHelper.getWritableDatabase(); ContentValues values = new ContentValues(); values.put("name", "zhangsan"); values.put("tel", "110"); // 返回新行的id long insert = db.insert("info", null, values); db.close(); if (insert > 0) { Toast.makeText(getApplicationContext(), "add 成功", Toast.LENGTH_LONG) .show(); } else { Toast.makeText(getApplicationContext(), "add 失败", Toast.LENGTH_LONG) .show(); } } // 删 public void click2(View v) { SQLiteDatabase db = myOpenHelper.getWritableDatabase(); // 返回值:影响的行数 int delete = db.delete("info", "name=?", new String[] { "zhangsan" }); db.close(); Toast.makeText(getApplicationContext(), "删除了" + delete + "行", Toast.LENGTH_LONG).show(); } // 改 public void click3(View v) { SQLiteDatabase db = myOpenHelper.getWritableDatabase(); ContentValues values = new ContentValues(); values.put("tel", "666"); // 返回值:更新了多少行 int update = db.update("info", values, "name=?", new String[] { "zhangsan" }); db.close(); Toast.makeText(getApplicationContext(), "更新了" + update + "行", Toast.LENGTH_LONG).show(); } // 查 public void click4(View v) { SQLiteDatabase db = myOpenHelper.getWritableDatabase(); Cursor cursor = db.query("info", new String[] { "tel" }, "name=?", new String[] { "zhangsan" }, null, null, null); if (cursor != null && cursor.getCount() > 0) { while (cursor.moveToNext()) { String tel = cursor.getString(0); System.out.println(tel); } } db.close(); } }
事务操作:执行一段逻辑,要么同时成功,要么同时失败
这里写一个逻辑转账的案例
package org.dreamtech.transaction; import android.content.Context; import android.database.sqlite.SQLiteDatabase; import android.database.sqlite.SQLiteOpenHelper; public class MyOpenHelper extends SQLiteOpenHelper { public MyOpenHelper(Context context) { super(context, "Account.db", null, 1); } @Override public void onCreate(SQLiteDatabase db) { db.execSQL("create table info(_id integer primary key autoincrement,name varchar(20),money varchar(20))"); db.execSQL("insert into info('name','money') values('a','2000')"); db.execSQL("insert into info('name','money') values('b','5000')"); } @Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { } }
package org.dreamtech.transaction; import android.os.Bundle; import android.app.Activity; import android.database.sqlite.SQLiteDatabase; import android.view.Menu; import android.view.View; import android.widget.Toast; public class MainActivity extends Activity { private MyOpenHelper myOpenHelper; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); myOpenHelper = new MyOpenHelper(getApplicationContext()); } // 使用事务进行转账 public void click(View v) { SQLiteDatabase db = myOpenHelper.getReadableDatabase(); // 开启事务 db.beginTransaction(); try { // 转账逻辑 db.execSQL("update info set money=money-100 where name=?", new Object[] { "a" }); // 加入这一行则不会成功 // int a = 10/0; db.execSQL("update info set money=money+100 where name=?", new Object[] { "b" }); // 表示当前事务成功(自动提交) db.setTransactionSuccessful(); } catch (Exception e) { Toast.makeText(getApplicationContext(), "转账失败", Toast.LENGTH_LONG) .show(); } finally { // 关闭事务 db.endTransaction(); } } }
加入10/0一行的效果: