在android中更新数据库版本时，Sqlite数据库数据中的问题清除

Question

朋友您好我是通过编程方式在我的应用程序中创建SQLite数据库。

这是我的代码：

    public class DatabaseHandler extends SQLiteOpenHelper {

    // All Static variables
    // Database Version
    private static final int DATABASE_VERSION =3;

    // Database Name
    private static final String DATABASE_NAME = "contactsManager";

    // Contacts table name
    private static final String TABLE_CONTACTS = "contacts";

    // Contacts Table Columns names
    private static final String KEY_ID = "id";
    private static final String KEY_NAME = "name";
    private static final String KEY_PH_NO = "phone_number";
    private static final String KEY_EMAIL = "email";
    private static final String KEY_ADD = "address";

    private final ArrayList<Contact> contact_list = new ArrayList<Contact>();

    public DatabaseHandler(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    // Creating Tables
    @Override
    public void onCreate(SQLiteDatabase db) {
    String CREATE_CONTACTS_TABLE = "CREATE TABLE " + TABLE_CONTACTS + "("
        + KEY_ID + " INTEGER PRIMARY KEY," + KEY_NAME + " TEXT,"
        + KEY_PH_NO + " TEXT," + KEY_EMAIL + " TEXT,"+ KEY_ADD + " TEXT  )";
    db.execSQL(CREATE_CONTACTS_TABLE);
        System.out.println("Create");
    }

    // Upgrading database
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // Drop older table if existed
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_CONTACTS);

        // If you need to add a column
        if (newVersion > oldVersion) {

            onCreate(db);
            db.execSQL("ALTER TABLE contacts ADD COLUMN gender TEXT;");

        }
    // Create tables again

    }

    /**
     * All CRUD(Create, Read, Update, Delete) Operations
     */

    // Adding new contact
    public void Add_Contact(Contact contact) {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues values = new ContentValues();
    values.put(KEY_NAME, contact.getName()); // Contact Name
    values.put(KEY_PH_NO, contact.getPhoneNumber()); // Contact Phone
    values.put(KEY_EMAIL, contact.getEmail()); // Contact Email
    // Inserting Row
    db.insert(TABLE_CONTACTS, null, values);
    db.close(); // Closing database connection
    }

    // Getting single contact
    Contact Get_Contact(int id) {
    SQLiteDatabase db = this.getReadableDatabase();

    Cursor cursor = db.query(TABLE_CONTACTS, new String[] { KEY_ID,
        KEY_NAME, KEY_PH_NO, KEY_EMAIL }, KEY_ID + "=?",
        new String[] { String.valueOf(id) }, null, null, null, null);
    if (cursor != null)
        cursor.moveToFirst();

    Contact contact = new Contact(Integer.parseInt(cursor.getString(0)),
        cursor.getString(1), cursor.getString(2), cursor.getString(3));
    // return contact
    cursor.close();
    db.close();

    return contact;
    }

    // Getting All Contacts
    public ArrayList<Contact> Get_Contacts() {
    try {
        contact_list.clear();

        // Select All Query
        String selectQuery = "SELECT  * FROM " + TABLE_CONTACTS;

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to list
        if (cursor.moveToFirst()) {
        do {
            Contact contact = new Contact();
            contact.setID(Integer.parseInt(cursor.getString(0)));
            contact.setName(cursor.getString(1));
            contact.setPhoneNumber(cursor.getString(2));
            contact.setEmail(cursor.getString(3));
            // Adding contact to list
            contact_list.add(contact);
        } while (cursor.moveToNext());
        }

        // return contact list
        cursor.close();
        db.close();
        return contact_list;
    } catch (Exception e) {
        // TODO: handle exception
        Log.e("all_contact", "" + e);
    }

    return contact_list;
    }

    // Updating single contact
    public int Update_Contact(Contact contact) {
    SQLiteDatabase db = this.getWritableDatabase();

    ContentValues values = new ContentValues();
    values.put(KEY_NAME, contact.getName());
    values.put(KEY_PH_NO, contact.getPhoneNumber());
    values.put(KEY_EMAIL, contact.getEmail());

    // updating row
    return db.update(TABLE_CONTACTS, values, KEY_ID + " = ?",
        new String[] { String.valueOf(contact.getID()) });
    }

    // Deleting single contact
    public void Delete_Contact(int id) {
    SQLiteDatabase db = this.getWritableDatabase();
    db.delete(TABLE_CONTACTS, KEY_ID + " = ?",
        new String[] { String.valueOf(id) });
    db.close();
    }

    // Getting contacts Count
    public int Get_Total_Contacts() {
    String countQuery = "SELECT  * FROM " + TABLE_CONTACTS;
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery(countQuery, null);
    cursor.close();

    // return count
    return cursor.getCount();
    }

   }

问题是当我将数据库版本从1升级到2时，我的数据库中添加了新列，但我的旧数据被删除了，那么我该如何解决这个问题呢？你的所有建议都很明显。

修改

Ya为 Rajesh Jadav 和 Amy 的解决方案工作但是当我将版本2更改为3时，它会给出错误，例如重复的列性别。我的功能如下

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// Drop older table if existed
//db.execSQL("DROP TABLE IF EXISTS " + TABLE_CONTACTS);

    // If you need to add a column
    if (newVersion > oldVersion) {


        db.execSQL("ALTER TABLE contacts ADD COLUMN gender TEXT;");
        db.execSQL("ALTER TABLE contacts ADD COLUMN dob TEXT;");

    }


}

但是当我对第db.execSQL("ALTER TABLE contacts ADD COLUMN gender TEXT;");行进行评论时，它的工作正常，那么每当我升级数据库版本时，有什么正确的方法可以在那个地方进行评论吗？

Answer 1

试试这个。

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

    // If you need to add a column
    if (newVersion > oldVersion) {

        db.execSQL("ALTER TABLE contacts ADD COLUMN gender TEXT;");

    }
// Create tables again

}

您已在onUpgrade中调用 db.execSQL（“DROP TABLE IF EXISTS”+ TABLE_CONTACTS）; 。所以它将清除所有现有数据。

如果你必须保留旧数据删除该查询。

修改

重复列问题有两种解决方案。

<强>首先

你可以通过try catch语句包围。

try{
      db.execSQL("ALTER TABLE contacts ADD COLUMN gender TEXT;");
       db.execSQL("ALTER TABLE contacts ADD COLUMN dob TEXT;");
   }catch(Exception e){
 }

<强>第二

您可以使用此方法检查表中是否已存在列。

private boolean existsColumnInTable(SQLiteDatabase inDatabase, String inTable, String columnToCheck) {
    Cursor mCursor = null;
    try {
        // Query 1 row 
        mCursor = inDatabase.rawQuery("SELECT * FROM " + inTable + " LIMIT 0", null);

        // getColumnIndex() gives us the index (0 to ...) of the column - otherwise we get a -1
        if (mCursor.getColumnIndex(columnToCheck) != -1)
            return true;
        else
            return false;

    } catch (Exception Exp) {
        // Something went wrong. Missing the database? The table?
        Log.d("... - existsColumnInTable", "When checking whether a column exists in the table, an error occurred: " + Exp.getMessage());
        return false;
    } finally {
        if (mCursor != null) mCursor.close();
    }
}

只需致电：

boolean gender_column_exist = existsColumnInTable(db,"contacts","gender");
if(!gender_column_exist){
db.execSQL("ALTER TABLE contacts ADD COLUMN gender TEXT;");
}
boolean dob_column_exist = existsColumnInTable(db,"contacts","dob");
if(!dob_column_exist){
db.execSQL("ALTER TABLE contacts ADD COLUMN dob TEXT;");
}

希望这有帮助！

Answer 2

试试这个：

 @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // Drop older table if existed
   // db.execSQL("DROP TABLE IF EXISTS " + TABLE_CONTACTS); //Remove This Line

        // If you need to add a column
        if (newVersion > oldVersion) {

            onCreate(db);
            db.execSQL("ALTER TABLE contacts ADD COLUMN gender TEXT;");

        }
    }

从onUpgrade（）方法中删除或注释第一行。它会删除旧表和数据。