android studio从数据库读取数据

可以通过 SQLiteOpenHelper 类来连接数据库并打开一个读取器（ReadableDatabase），然后使用 Cursor 对象进行查询操作，最后关闭读取器。具体实现可以参考以下代码：

首先定义一个数据库帮助类 DatabaseHelper，继承自 SQLiteOpenHelper：

public class DatabaseHelper extends SQLiteOpenHelper {
    // 数据库名称
    private static final String DB_NAME = "my_database.db";
    // 数据库版本号
    private static final int DB_VERSION = 1;
    // 数据表名称
    public static final String TABLE_NAME = "my_table";
    // 列名称
    public static final String COLUMN_ID = "id";
    public static final String COLUMN_NAME = "name";
    public static final String COLUMN_AGE = "age";

    public DatabaseHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // 创建数据表
        String sql = "CREATE TABLE " + TABLE_NAME + " ("
                + COLUMN_ID + " INTEGER PRIMARY KEY,"
                + COLUMN_NAME + " TEXT,"
                + COLUMN_AGE + " INTEGER)";
        db.execSQL(sql);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // 数据库升级时执行的操作
    }
}

然后在 MainActivity 中读取数据：

public class MainActivity extends AppCompatActivity {

    private SQLiteDatabase db;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 创建或打开数据库
        DatabaseHelper dbHelper = new DatabaseHelper(this);
        db = dbHelper.getReadableDatabase();

        // 查询数据
        String[] projection = {
                DatabaseHelper.COLUMN_ID,
                DatabaseHelper.COLUMN_NAME,
                DatabaseHelper.COLUMN_AGE
        };
        Cursor cursor = db.query(DatabaseHelper.TABLE_NAME, projection, null, null, null, null, null);

        // 遍历数据
        if (cursor.moveToFirst()) {
            do {
                int id = cursor.getInt(cursor.getColumnIndexOrThrow(DatabaseHelper.COLUMN_ID));
                String name = cursor.getString(cursor.getColumnIndexOrThrow(DatabaseHelper.COLUMN_NAME));
                int age = cursor.getInt(cursor.getColumnIndexOrThrow(DatabaseHelper.COLUMN_AGE));
                Log.d("MainActivity", "id = " + id + ", name = " + name + ", age = " + age);
            } while (cursor.moveToNext());
        }

        // 关闭资源
        cursor.close();
        db.close();
    }
}

注意要在 AndroidManifest.xml 中申请访问 SQLite 数据库的权限：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>