从现有的sqlite数据库中获取数据

Question

I am having an existing sqlite database. I am developing an android app and I want to connect it with this existing sqlite DataBase.

Problem 1: I have already included the sqlite database in my project via "DDMS push database function" as per my instructor's advise. Now I want to fetch the data from database, do I need to use SQLiteOpenHelper. If yes, how to use it and what will be coded in onCreate(SQLiteDatabase db) function and onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) function as we already have the Database with us, we don't really need to create it.

Problem 2: What should be done to simply fetch the required data from the existing database.

Being a newbie, I am quite confused, can someone please explain these concepts and guide me to overcome this issue. Any help would be appreciated.

I have seen a tutorial also for this purpose as sugggested by @TronicZomB, but according to this tutorial ( http://www.reigndesign.com/blog/using-your-own-sqlite-database-in-android-applications/ ), I must be having all the tables with primary key field as _id.

I have 7 tables namely destinations, events, tour, tour_cat, tour_dest, android_metadata and sqlite_sequence. Out of all, only tour_dest is not fulfilling the conditions of having a primary key named as _id. How to figure out this one?

Following is the screenshot of table which is lacking the primary key field necessary for binding id fields of database tables.

Answer 1

The onCreate and onUpgrade methods will be empty since you already have the database. There is a great tutorial on how to achieve this here .

You could then access the database like such (example):

public ArrayList<String> getValues(String table) {
    ArrayList<String> values = new ArrayList<String>();

    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery("SELECT value FROM " + table, null);

    if(cursor.moveToFirst()) {
        do {
            values.add(cursor.getString(cursor.getColumnIndex("value")));
        }while(cursor.moveToNext());
    }

    cursor.close();
    db.close();
    return values;
}

Answer 2

除非您对查询，数据库等非常满意，否则我强烈建议您使用http://satyan.github.io/sugar/ ，它还会删除很多在Android中执行sqlite所需的样板代码

Answer 3

1. If DB already exists, onCreate will not invoke. onUpgrade will be invoked only if you will change DB version. onUpgrade you should to use if there some changes in your APP's database, and you have to make migration on new structure of data smoothly.

public class DbInit extends SQLiteOpenHelper {
    private static final String DATABASE_NAME = "name";
    private static final int DATABASE_VERSION = 3;
    private static final String DATABASE_CREATE = "create table  connections  . .. . ...

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

    @Override
    public void onCreate(SQLiteDatabase database) {
        database.execSQL(DATABASE_CREATE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
         if (isChangeFromToVersion(1, 2, oldVersion, newVersion)) {
             //Execute UPDATE here
         }
    }

    private boolean isChangeFromToVersion(int from, int to, int oldVersion, int newVersion ) {
        return (from == oldVersion && to == newVersion);
  }
....

2. Simple example how to open connection to DB and get cursor object.

public class DAO {

private SQLiteDatabase database;
private DbInit dbHelper;

public ConnectionDAO(Context context) {
    dbHelper = new DbInit(context);
}

public void open() throws SQLException {
    database = dbHelper.getWritableDatabase();
}

public Connection getConnectionById(long id) {
    Cursor cursor = null;
    try {
        open();
        cursor = database.query(DbInit.TABLE_CONNECTIONS, allColumns, DbInit.COLUMN_ID + " = '" + id + "'", null, null, null, null);
        if (!cursor.moveToFirst())
            return null;
        return cursorToConnection(cursor);
    } finally {
        if (cursor != null)
            cursor.close();
        close();
    }
}

private Connection cursorToConnection(Cursor cursor) {
    Connection connection = new Connection();
    connection.setId(cursor.isNull(0) ? null : cursor.getInt(0));
    connection.setName(cursor.isNull(1) ? null : cursor.getString(1));
    .....
    .....
    return connection;
}