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在Android中使用SQLite数据库进行查询操作时,可以使用SQLiteDatabase类中的query()方法来执行查询操作。以下是一个简单的示例代码:
// 创建SQLiteOpenHelper的子类来管理数据库的创建和版本升级 public class DatabaseHelper extends SQLiteOpenHelper { public static final String DATABASE_NAME = "mydatabase.db"; public static final int DATABASE_VERSION = 1; public static final String TABLE_NAME = "mytable"; public static final String COLUMN_ID = "_id"; public static final String COLUMN_NAME = "name"; public DatabaseHelper(Context context) { super(context, DATABASE_NAME, null, DATABASE_VERSION); } @Override public void onCreate(SQLiteDatabase db) { String createTableQuery = "CREATE TABLE " + TABLE_NAME + "(" + COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + COLUMN_NAME + " TEXT)"; db.execSQL(createTableQuery); } @Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME); onCreate(db); } public Cursor queryData() { SQLiteDatabase db = this.getReadableDatabase(); String[] projection = { COLUMN_ID, COLUMN_NAME }; Cursor cursor = db.query( TABLE_NAME, projection, null, null, null, null, null ); return cursor; } }
在上面的代码中,首先创建一个DatabaseHelper类继承自SQLiteOpenHelper类,其中定义了数据库名称、版本、表名和列名等常量。在onCreate()方法中创建表格,然后在queryData()方法中执行查询操作。
在Activity中使用该DatabaseHelper类进行查询操作:
DatabaseHelper dbHelper = new DatabaseHelper(this); Cursor cursor = dbHelper.queryData(); if (cursor != null && cursor.moveToFirst()) { do { int id = cursor.getInt(cursor.getColumnIndex(DatabaseHelper.COLUMN_ID)); String name = cursor.getString(cursor.getColumnIndex(DatabaseHelper.COLUMN_NAME)); Log.d("Query Result", "ID: " + id + ", Name: " + name); } while (cursor.moveToNext()); } cursor.close(); dbHelper.close();
在Activity中创建DatabaseHelper对象,并调用queryData()方法获取Cursor对象,然后遍历Cursor对象获取查询结果。最后关闭Cursor和DatabaseHelper对象。
请注意在使用完Cursor对象和DatabaseHelper对象后记得调用close()方法释放资源。