Question

经过大量的工作，我设法建立了我的第一个应用程序，但我坚持一个问题。对于我的应用程序，我使用的是SQL数据库..假设我想在某个表中添加30条记录。当我在Android市场上推出一个新版本的sql表以便将来使用这个版本，但保留以前数据库的记录时，怎么可能呢？

是否必须采取以下措施：

@Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    }

修改

我的databasehelper代码：

 public class DataBaseHelper extends SQLiteOpenHelper {


    private static String DB_PATH = "/data/data/com.test.com/databases/";
    private static String DB_NAME = "quizDb";
    private SQLiteDatabase myDataBase;
    private final Context myContext;
    private Cursor c;
    static int numberOfLevels = 10;
    private final static int DB_VERSION = 2; // = until level 10


    /**
     * Constructor Takes and keeps a reference of the passed context in order to
     * access to the application assets and resources.
     * 
     * @param context
     */
    public DataBaseHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
        this.myContext = context;

    }

    /**
     * Creates a empty database on the system and rewrites it with your own
     * database.
     * */
    public void createDataBase() throws IOException {

        boolean dbExist = checkDataBase();
        if (!dbExist) {
            // By calling this method and empty database will be created into
            // the default system path
            // of your application so we are gonna be able to overwrite that
            // database with our database.
            this.getReadableDatabase();

            try {
                copyDataBase();
            } catch (IOException e) {
                throw new Error("Error copying database");
            }
        }
    }

    /**
     * Check if the database already exist to avoid re-copying the file each
     * time you open the application.
     * 
     * @return true if it exists, false if it doesn't
     */
    private boolean checkDataBase() {
        File dbFile = new File(DB_PATH + DB_NAME);
        return dbFile.exists();
    }

    /**
     * Copies your database from your local assets-folder to the just created
     * empty database in the system folder, from where it can be accessed and
     * handled. This is done by transfering bytestream.
     * */
    private void copyDataBase() throws IOException {

        // Open your local db as the input stream
        InputStream myInput = myContext.getAssets().open(DB_NAME);

        // Path to the just created empty db
        String outFileName = DB_PATH + DB_NAME;

        // Open the empty db as the output stream
        OutputStream myOutput = new FileOutputStream(outFileName);

        // transfer bytes from the inputfile to the outputfile
        byte[] buffer = new byte[1024];
        int length;
        while ((length = myInput.read(buffer)) > 0) {
            myOutput.write(buffer, 0, length);
        }

        // Close the streams
        myOutput.flush();
        myOutput.close();
        myInput.close();

    }

    public void openDataBase() throws SQLException {
        // Open the database
        String myPath = DB_PATH + DB_NAME;

        myDataBase = SQLiteDatabase.openDatabase(myPath, null,
                SQLiteDatabase.OPEN_READONLY);
    }

    @Override
    public synchronized void close() {
        if (c != null)
            c.close();
        if (myDataBase != null)
            myDataBase.close();

        super.close();
    }

    @Override
    public void onCreate(SQLiteDatabase db) {

    }

    public File getDatabasePath(String name) {

        File file = myContext.getDatabasePath(name);

        return file;
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("ATTACH DATABASE ? as AttachedDB",
                new String[] { getDatabasePath("quizDbNew").getPath() });
        db.execSQL("INSERT OR IGNORE INTO questions (_id, file, answer, level) SELECT _id, file, answer, level FROM AttachedDB.questions");
        db.execSQL("DETACH AttachedDB");

    }

Answer 1

在"DROP TABLE"中使用onUpgrade()的概念与数据库管理一样原始，但更有用的技术需要更多的SQL精明。使用"ALTER TABLE"添加新列或以其他方式将旧数据导入新架构的更智能的升级数据库方法。

<强>加成
以下是您所说的评论（或多或少）：

我想将Db v1备份文件中的内容复制到我当前的Db v2
中

所以，让我们设置几个假设的表格：

数据库版本1（DBv1）：

CREATE TABLE Foo(_id INTEGER PRIMARY KEY, bar TEXT, bar2 TEXT, bar3 TEXT);

数据库版本二（DBv2）：

CREATE TABLE Foo(_id INTEGER PRIMARY KEY, bar2 TEXT, bar4 INTEGER);

首先让我们看看从DBv1到DBv2的定期升级。 SQLite仅支持ADD COLUMN和RENAME TO，而不支持REMOVE COLUMN或其他任何方式。所以我们必须重新创建整个表：

@Override  // DBv1 => DBv2
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("ALTER TABLE Foo RENAME TO OldFoo");
    db.execSQL("CREATE TABLE Foo(_id INTEGER PRIMARY KEY, bar2 TEXT, bar4 INTEGER)");
    db.execSQL("INSERT INTO Foo (_id, bar) SELECT _id, bar2 FROM OldFoo");
    db.execSQL("DROP TABLE OldFoo");
}

这又创建了一个包含DBv2模式的表，并通过将适当的列插入DBv2来保留DBv1中的所有有效现有数据。（然后通过删除旧表删除旧数据。）

您明智地选择在单独的文件中备份数据库，但现在您要将旧数据引入新的表模式。要开始，请确保备份SQLite文件与当前SQLite文件（data/data/<reverse.package.name>/databases/）位于同一目录中。它显然需要一个唯一的名称，我们称之为DBBackup。现在让我们将DBBackup附加到您当前的数据库，并从上面执行类似的操作：

// DBBackupv1 => DBv2
public void restore(SQLiteDatabase db) {
    db.execSQL("ATTACH DATABASE ? as AttachedDB", new String[] {getDatabasePath("DBBackup").getPath()});
    db.execSQL("INSERT OR IGNORE INTO Foo (_id, bar2) SELECT _id, bar2 FROM AttachedDB.Foo");
    db.execSQL("DETACH AttachedDB");
}

我使用INSERT OR IGNORE来恢复已删除的所有行但保持当前现有行不变。您可以使用INSERT OR REPLACE还原到备份版本。还有更多选择可满足您的需求。

替换sql数据库但保留旧记录

1 个答案: