Question

我在Android应用程序中创建了这个类。但是，当我要从创建的数据库中检索数据时，它显示数据库不可用。请帮我这样做。感谢

public class DatabaseHandler extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "dbumbers";
      private static final int DATABASE_VERSION = 1;

      private static final String TABLE_NAME = "numbers";

    public DatabaseHandler(Context context, String name, CursorFactory factory,
            int version) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);

    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        //Create database query
        db.execSQL("create table " + TABLE_NAME + " (district TEXT, category TEXT, description TEXT, number TEXT); ");

        //Insert query
        db.execSQL("insert into " + TABLE_NAME + " values(Ampara,hospital,Ampara General Hospital,0112456852);");
        db.execSQL("insert into " + TABLE_NAME + " values(Anuradhapura,bar,Anuradhapura Bar,011245852);");
        db.execSQL("insert into " + TABLE_NAME + " values(Colombo,retail,Colombo Supermarket,0112546852);");
        db.execSQL("insert into " + TABLE_NAME + " values(Gampaha,flight,Gampaha Airport,0112455552);");

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        //add more insert query if you need to add more datas after, but you have first to upgrade your DATABASE_VERSION to a higher number

    }

}

这是检索部分

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_data_check);

        SQLiteDatabase db = openOrCreateDatabase("dbumbers", 1, null);

        Cursor c = db.rawQuery("SELECT * FROM numbers", null);

        //c.moveToFirst();
        while (c.moveToNext()) {
            String district = c.getString(0);
            String category = c.getString(1);
            String description = c.getString(2);
            String number = c.getString(2);

            String row = district + "-" + category + "-" + description + "-" + number;

            Toast.makeText(this, row, Toast.LENGTH_LONG).show();

        }


        db.close();



    }

Answer 1

您应该使用DatabaseHandler打开数据库，而不是使用openOrCreateDatabase。像这样更改构造函数（因为您只需要其中一个参数）：

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

然后得到一个像这样的数据库：

DatabaseHandler dbHandler = new DatabaseHandler(this /*context*/);
SQLiteDatabase db = dbHandler.getReadableDatabase();
Cursor c = db.rawQuery(...);

Answer 2

尝试将数据库名称更改为：

private static final String DATABASE_NAME = "dbumbers.db";

Answer 3

你可以做这样的事情来使用SQLite

1）创建一个将扩展SQLieOpenHelper

的databaseHandler类

public class EventRegister extends SQLiteOpenHelper{

private static Context context;
//Database Version
private static final int DATABASE_VERSION=6;

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

//Table Names
private static final String TABLE_NAME="event_table";

//Database Constants I am taking some constants as demo you can take   your own
private static final String COLUMN_ID="id";
private static final String COLUMN_TITLE="event_title";
private static final String COLUMN_CATEGORY="event_category";
private static final String COLUMN_DATE="event_date";
private static final String COLUMN_TIME="event_time";
private static final String COLUMN_DESCRIPTION="event_description";


//Command to create a Table
private static final String CREATE_TABLE="create table "+TABLE_NAME+"("
        +COLUMN_ID+" integer primary key autoincrement,"
        +COLUMN_TITLE+" text,"
        +COLUMN_CATEGORY+" text,"
        +COLUMN_DATE+" text,"
        +COLUMN_TIME+" text,"
        +COLUMN_DESCRIPTION+" text )";



public EventRegister(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    this.context=context;


}


/*
 *This method will be called on Start
 */
@Override
public void onCreate(SQLiteDatabase db) {

    db.execSQL(CREATE_TABLE);
}


/*
 * This method will be called onUpgradate of Database
 */

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) 
{
    db.execSQL("DROP TABLE IF EXISTS "+TABLE_NAME);
    onCreate(db);
}


    //Method for adding data in database EventData is my Model Object You can insert value as Single or Using Model class also
    public void addEvent(EventData taskdata)
{
            int flag=0;
    SQLiteDatabase db=this.getWritableDatabase();

    ContentValues contentValues=new ContentValues();

    contentValues.put(COLUMN_TITLE, taskdata.getEvent_title());
    contentValues.put(COLUMN_CATEGORY, taskdata.getEvent_category());
    contentValues.put(COLUMN_DATE, taskdata.getEvent_date());
    contentValues.put(COLUMN_TIME, taskdata.getEvent_time());
    contentValues.put(COLUMN_DESCRIPTION, taskdata.getEvent_description());


    flag=db.insert(TABLE_NAME, null, contentValues);
    db.close();
}


     //Add Other methods here

从MainActivity或者你想获取数据可以使用someThing，如

EventRegister eventRegister=new EventRegister(context);
eventRegister.addData(EventData);

我希望你有一些指导如何使用SQLite

Answer 4

对于db操作，我使用这样的辅助类：

public class DBAdapter {
private static final String DATABASE_NAME = "DBNAME";
private static final int DATABASE_VERSION = 1;

private static DBAdapter sInstance;

private final Context mContext; 

public DatabaseHelper DBHelper;
public SQLiteDatabase mDb;

private DBAdapter(Context ctx)     {
    this.mContext = ctx.getApplicationContext();
    DBHelper = new DatabaseHelper(mContext);
} 

public static DBAdapter getInstance(Context context) {
    if (sInstance == null) {
      sInstance = new DBAdapter(context.getApplicationContext());
    }
    return sInstance;
  }

public static class DatabaseHelper extends SQLiteOpenHelper {
    DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        ...
    }

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



 }    

//---opens the database---
public DBAdapter open() throws SQLException {
    mDb = DBHelper.getWritableDatabase();
    return this;
}
//add any db functions you want here


 public Cursor getCursor(int var) {
    ...do stuff here with the mDb database
}

这使用单例模式，因此始终只使用与db的一个连接。

从活动中，我像这样访问数据库：

DBAdapter mDb = DBAdapter.getInstance(getApplicationContext());  
mDb.open();
    Cursor cur= mDb.getCursor(2);

您不需要关闭数据库，因为只使用一个连接，当应用程序关闭时它将关闭，但您也可以在open（）函数之后添加此连接

public void close() {
    DBHelper.close();
}

SQLite数据库不是由SQLite Open Helper创建的

4 个答案: