android - 从 SQLite 数据库 Android SDK 中获取随机行

Question

我想从我创建的数据库中随机获取一行，但我不知道该怎么做。

我在 StackOverflow 上发现了几个问题和答案，它们说我应该使用这个:

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

但是，我认为我有某种其他数据库，因为它返回一个不知道方法“查询”的错误。

这是我使用的数据库代码:

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);
        }
    }
}

有没有一种方法可以实现这个数据库类的“查询”方法？如果可以，如何实现？或者有人建议使用其他数据库代码吗？

编辑:

我可能没有在这里问正确的问题......我的问题是此时使用答案中给出的任何方法，例如

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

返回未定义的错误。

那么我需要在数据库适配器代码中更改什么才能正确使用此代码？

Answer 1

尝试删除this 并确保您的db 是SQLiteDatabase。您也可以通过以下方式获取随机行:

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

顺便说一句，您的代码中已经有了带有正确查询的 getRowRandom 方法。