了解Android.com&amp；s&amp；在SQL数据库中保存数据教程

问题描述

我正在阅读本教程：http://developer.android.com/training/basics/data-storage/databases.html，它提出了很多问题。

这是我在学习本教程时编写的代码：

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.provider.BaseColumns;

public final class SongDbHelper extends SQLiteOpenHelper {
    // If you change the database schema, you must increment the database version.
    public static final int DATABASE_VERSION = 1;
    public static final String DATABASE_NAME = "Song.db";
    private static final String TEXT_TYPE = " TEXT";
    private static final String PRIMARY_KEY_TYPE = " INTEGER PRIMARY KEY";
    private static final String COMMA_SEP = ",";
    private static final String SQL_CREATE_ENTRIES =
            "CREATE TABLE " + SongEntry.TABLE_NAME + " (" +
                    SongEntry._ID + PRIMARY_KEY_TYPE + COMMA_SEP +
                    SongEntry.COLUMN_NAME_SONG_TITLE + TEXT_TYPE + COMMA_SEP +
                    SongEntry.COLUMN_NAME_ENGLISH_LYRICS + TEXT_TYPE + COMMA_SEP +
                    SongEntry.COLUMN_NAME_ROMAJI_LYRICS + TEXT_TYPE + COMMA_SEP +
                    SongEntry.COLUMN_NAME_KANJI_LYRICS + TEXT_TYPE + COMMA_SEP +
                    SongEntry.COLUMN_NAME_INFO + TEXT_TYPE +
            " )";
    private static final String SQL_DELETE_ENTRIES =
            "DROP TABLE IF EXISTS " + SongEntry.TABLE_NAME;

    public SongDbHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(SQL_CREATE_ENTRIES);
    }
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // This database is only a cache for online data, so its upgrade policy is
        // to simply to discard the data and start over
        db.execSQL(SQL_DELETE_ENTRIES);
        onCreate(db);
    }
    public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        onUpgrade(db, oldVersion, newVersion);
    }


    public static abstract class SongEntry implements BaseColumns {
        public static final String TABLE_NAME = "Song";
        public static final String COLUMN_NAME_SONG_TITLE = "c29uZ190aXRsZQ==";
        public static final String COLUMN_NAME_ENGLISH_LYRICS = "english_lyrics";
        public static final String COLUMN_NAME_ROMAJI_LYRICS = "romaji_lyrics";
        public static final String COLUMN_NAME_KANJI_LYRICS = "kanji_lyrics";
        public static final String COLUMN_NAME_INFO = "info";

    }
}

1. 我们是否应该在应用每次打开时都调用SongDbHelper.onCreate()？(我设想将其放入我的主活动的onCreate()方法中。

2. 如果您多次调用SongDbHelper.onCreate()，它是重新创建表，还是SongDbHelper.onCreate()只是静默正常失败？

3. APP为什么要调用SongDbHelper.onCreate()呢？如果我将我的应用放到应用商店上，难道我不会只让人们下载我已经填满数据的数据库吗？

4. 何时/在我的应用程序中调用SongDbHelper.onUpgrade()？我假设当用户注意到"这个应用程序有可用的更新。点击这里下载它"或类似的东西时，就会发生这种情况。我只是感到困惑，因为我假设用户只是下载了一个数据库完好无损的.db文件，因此不需要调用SongDbHelper.onCreate()。

5. 如果APP必须调用SongDbHelper.onCreate()，它如何在数据库中填充值？我认为如果我还有填充值的代码，这将违背数据库的目的，因为这样值就会重复(在代码和数据库中)。

6. 向数据库填充值的最佳方式是什么？使用终端，或以编程方式在应用程序之外的单独程序中，还是仅在Android应用程序中？

推荐答案

答案：

SongDbHelper mDbHelper = new SongDbHelper(getContext());
// this will call super method internally and
// this will create table in database