我想从我创建的数据库中获取随机行,但是我不知道该怎么做。
我在StackOverflow上找到了几个问题和答案,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,因为它不知道方法“查询”会返回错误。
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该数据库类的“查询”方法,如果可以,如何实现?还是有人建议使用其他数据库代码?
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);
返回一个未定义的错误。
那么,为了正确使用此代码,我需要更改数据库适配器代码中的什么?
尝试删除this
并确保您db
是SQLiteDatabase。您还可以通过以下方式获得随机行:
db.rawQuery("SELECT * FROM mainTable ORDER BY RANDOM() LIMIT 1", null);
顺便说一句,您getRowRandom
的代码中已经有正确查询的方法。
本文收集自互联网,转载请注明来源。
如有侵权,请联系[email protected] 删除。
我来说两句