從SQLite數據庫Android獲得隨機行Android SDK

Question

我想從我創建的數據庫中獲取一個隨機行，但我無法弄清楚如何去做。

我發現了幾個問題和答案在這裏StackOverflow上哪說我應該使用這樣的：

Cursor cursor = this.db.query("mainTable Order BY RANDOM() LIMIT 1", 
      new String[] { "*" }, null, null, null, null, null); 

不過，我覺得我有某種其他的數據庫，因爲它返回一個錯誤，不知道的方法「查詢」 。

這是數據庫的代碼，我使用：

package com.example.database; 

import android.content.ContentValues; 
import android.content.Context; 
import android.database.Cursor; 
import android.database.sqlite.SQLiteDatabase; 
import android.database.sqlite.SQLiteOpenHelper; 
import android.util.Log; 

public class DBAdapter{ 

    private static final String TAG = "DBAdapter"; 

    public static final String KEY_ROWID = "_id"; 
    public static final int COL_ROWID = 0; 

    public static final String KEY_PIC = "picture"; 
    public static final String KEY_ANS = "answer"; 

    public static final int COL_PIC = 1; 
    public static final int COL_ANS = 2; 


    public static final String[] ALL_KEYS = new String[] {KEY_ROWID, KEY_PIC, KEY_ANS}; 

    public static final String DATABASE_NAME = "MyDb"; 
    public static final String DATABASE_TABLE = "mainTable"; 
    public static final int DATABASE_VERSION = 2; 

    private static final String DATABASE_CREATE_SQL = 
      "create table " + DATABASE_TABLE 
      + " (" + KEY_ROWID + " integer primary key autoincrement, " 
      + KEY_PIC + " integer not null, " 
      + KEY_ANS + " text not null" 
      + ");"; 

    private final Context context; 

    private DatabaseHelper myDBHelper; 
    private SQLiteDatabase db; 

    public GevaarherkenningDBHelper(Context ctx) { 
     this.context = ctx; 
     myDBHelper = new DatabaseHelper(context); 
    } 

    public GevaarherkenningDBHelper open() { 
     db = myDBHelper.getWritableDatabase(); 
     return this; 
    } 

    public void close() { 
     myDBHelper.close(); 
    } 

    public long insertRow(int pic, String answer) { 
     ContentValues initialValues = new ContentValues(); 

     initialValues.put(KEY_PIC, pic); 
     initialValues.put(KEY_ANS, answer); 

     return db.insert(DATABASE_TABLE, null, initialValues); 
    } 

    public boolean deleteRow(long rowId) { 
     String where = KEY_ROWID + "=" + rowId; 
     return db.delete(DATABASE_TABLE, where, null) != 0; 
    } 

    public void deleteAll() { 
     Cursor c = getAllRows(); 
     long rowId = c.getColumnIndexOrThrow(KEY_ROWID); 
     if (c.moveToFirst()) { 
      do { 
       deleteRow(c.getLong((int) rowId));    
      } while (c.moveToNext()); 
     } 
     c.close(); 
    } 

    public Cursor getAllRows() { 
     String where = null; 
     Cursor c = db.query(true, DATABASE_TABLE, ALL_KEYS, 
          where, null, null, null, null, null); 
     if (c != null) { 
      c.moveToFirst(); 
     } 
     return c; 
    } 

    public Cursor getRow(long rowId) { 
     String where = KEY_ROWID + "=" + rowId; 
     Cursor c = db.query(true, DATABASE_TABLE, ALL_KEYS, 
         where, null, null, null, null, null); 
     if (c != null) { 
      c.moveToFirst(); 
     } 
     return c; 
    } 

    public Cursor getRowRandom(String rowId) { 
     Cursor c = db.query(true, DATABASE_TABLE, ALL_KEYS, 
         null, null, null, null, "RANDOM()", "1"); 
     if (c != null) { 
      c.moveToFirst(); 
     } 
     return c; 
    } 

    public boolean updateRow(long rowId, int pic, String answer) { 
     String where = KEY_ROWID + "=" + rowId; 

     ContentValues newValues = new ContentValues(); 
     newValues.put(KEY_PIC, pic); 

     newValues.put(KEY_ANS, answer); 

     return db.update(DATABASE_TABLE, newValues, where, null) != 0; 
    } 

    private static class DatabaseHelper extends SQLiteOpenHelper 
    { 
     DatabaseHelper(Context context) { 
      super(context, DATABASE_NAME, null, DATABASE_VERSION); 
     } 

     @Override 
     public void onCreate(SQLiteDatabase _db) { 
      _db.execSQL(DATABASE_CREATE_SQL);   
     } 

     @Override 
     public void onUpgrade(SQLiteDatabase _db, int oldVersion, int newVersion) { 
      Log.w(TAG, "Upgrading application's database from version " + oldVersion 
        + " to " + newVersion + ", which will destroy all old data!"); 

      _db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE); 

      onCreate(_db); 
     } 
    } 
} 

有沒有一種方法，我可以實現「query'法在此數據庫類，如果是這樣，如何？還是有人建議使用其他數據庫代碼？

編輯：

我可能還沒有問正確的問題在這裏...我的問題是，使用任何在該點的答案給出的方法，如

db.rawQuery("SELECT * FROM mainTable ORDER BY RANDOM() LIMIT 1", null); 

返回錯誤，它是未定義的。

那麼我需要在數據庫適配器代碼中更改以正確使用此代碼？

Answer 1

嘗試刪除this並確保您的db是SQLiteDatabase。您也可以通過以下方式獲得隨機行：

db.rawQuery("SELECT * FROM mainTable ORDER BY RANDOM() LIMIT 1", null); 

順便說一句，你已經在使用正確的查詢代碼getRowRandom方法。

Answer 2

query()是SQLiteDatabase上的方法。您的DBAdapter課程中已經使用了query()，例如您的getAllRows()方法。