Question

I’m having some troubles figure out how i should make my db query to get the cursor to select the right information.

My database is structured like this: https://i.stack.imgur.com/vPaoD.png (Can’t embed image because lack of rep)

Feed got a many-to-many relationship with user and a Item got a one-to-many relationship with user.

I want to create a method which returns all users or items that is in a specific feed with a method called getUsers(Feed feed).

My database code is:
public class DatabaseHelper extends SQLiteOpenHelper {

// Database static variables
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "feedlrDatabase";

// Declaring feed table
private static final String TABLE_FEED = "feed";
private static final String FEED_COLUMN_ID = "_id";
private static final String FEED_COLUMN_NAME = "name";

// Declaring feed-user bridge table
private static final String TABLE_FEEDUSER = "feeduser";
private static final String FEEDUSER_COLUMN_FEED_ID = "feed_ID";
private static final String FEEDUSER_COLUMN_USER_ID = "user_ID";

// Declaring user table
private static final String TABLE_USER = "user";
private static final String USER_COLUMN_ID = "_id";
public static final String USER_COLUMN_USERNAME = "username";
private static final String USER_COLUMN_USERID = "userid";
private static final String USER_COLUMN_IMGURL = "ProfileImageURL";
private static final String USER_COLUMN_SOURCE = "source";

// Declaring item table
private static final String TABLE_ITEM = "item";
public static final String ITEM_COLUMN_ID = "_id";
public static final String ITEM_COLUMN_TEXT = "text";
private static final String ITEM_COLUMN_TIMESTAMP = "timestamp";
private static final String ITEM_COLUMN_TYPE = "type";
private static final String ITEM_COLUMN_URL = "URL";
private static final String ITEM_COLUMN_IMGURL = "imgURL";
private static final String ITEM_COLUMN_USER_ID = "user_ID";

private SQLiteDatabase db = this.getWritableDatabase();

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

public void onCreate(SQLiteDatabase database) {
    // @formatter:off
    // Creating feed table
    database.execSQL("CREATE TABLE " + TABLE_FEED + "(" + FEED_COLUMN_ID
            + " INTEGER PRIMARY KEY AUTOINCREMENT," + FEED_COLUMN_NAME
            + " TEXT UNIQUE" + ")");

    // Creating feed-user bridge table
    database.execSQL("CREATE TABLE " + TABLE_FEEDUSER + "("
            + FEEDUSER_COLUMN_FEED_ID + " INT NOT NULL,"
            + FEEDUSER_COLUMN_USER_ID + " INT NOT NULL" + ")");

    // Creating user table
    // TODO Should username be the unique idenifier of a user?!
    database.execSQL("CREATE TABLE " + TABLE_USER + "(" + USER_COLUMN_ID
            + " INTEGER PRIMARY KEY AUTOINCREMENT," + USER_COLUMN_USERNAME
            + " TEXT NOT NULL," + USER_COLUMN_USERID + " TEXT UNIQUE,"
            + USER_COLUMN_IMGURL + " TEXT," + USER_COLUMN_SOURCE
            + " TEXT NOT NULL" + ")");

    // Creating item table
    database.execSQL("CREATE TABLE " + TABLE_ITEM + "(" + ITEM_COLUMN_ID
            + " INTEGER PRIMARY KEY AUTOINCREMENT," + ITEM_COLUMN_TEXT
            + " TEXT," + ITEM_COLUMN_TIMESTAMP + " TEXT,"
            + ITEM_COLUMN_TYPE + " TEXT," + ITEM_COLUMN_URL + " TEXT,"
            + ITEM_COLUMN_IMGURL + " TEXT," + ITEM_COLUMN_USER_ID
            + " INT NOT NULL" + ")");
    // @formatter:on
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // Temporarily drops all tables
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_FEED);
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_FEEDUSER);
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_USER);
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_ITEM);
    onCreate(db);
}

public void addFeed(Feed feed) {

    ContentValues temp = new ContentValues();

    temp.put(FEED_COLUMN_NAME, feed.getTitle());
    try {
        db.insertOrThrow(TABLE_FEED, null, temp);
    } catch (SQLiteConstraintException e) {
        Log.d("ERROR", "Inserted feed is not UNIQUE!");
        // TODO Apply listener to notify the user that the feed name already
        // exists!
    }

    // db.close();
}

public void removeFeed(Feed feed) {
    String title = feed.getTitle();
    long id = getFeedID(feed);

    removeFeedBridge(id);

    db.delete(TABLE_FEED, FEED_COLUMN_NAME + "=?", new String[] { title });
}

public ArrayList<String> listFeeds() {
    final ArrayList<String> feeds = new ArrayList<String>();

    Cursor c = db.rawQuery("SELECT * FROM " + TABLE_FEED, null);
    while (c.moveToNext()) {
        String s = c.getString(1);
        feeds.add(s);
    }
    c.close();
    return feeds;
}

public long getFeedID(Feed feed) {
    String feedTitle = feed.getTitle();
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor c = db.query(TABLE_FEED, new String[] { FEED_COLUMN_ID },
            FEED_COLUMN_NAME + "=?", new String[] { feedTitle }, null,
            null, null);
    c.moveToNext();
    Long id = Long.parseLong(c.getString(0));
    c.close();
    return id;
}

public long getUserID(User user) {
    long id = user.getId();
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor c = db.query(TABLE_USER, new String[] { USER_COLUMN_ID },
            USER_COLUMN_ID + "=?", new String[] { id + "" }, null, null,
            null);
    c.moveToNext();
    Long id1 = Long.parseLong(c.getString(0));
    c.close();
    return id1;
}

public ArrayList<String> listUsers() {
    final ArrayList<String> users = new ArrayList<String>();

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

    while (c.moveToNext()) {
        String s = c.getString(1);
        users.add(s);
    }
    return users;
}

public void addUserToFeed(User user, Feed feed) {
    long FeedID = getFeedID(feed);
    long UserID = addUser(user);

    // Add bridge connection
    if (UserID != -1) {
        addFeedUserBridge(FeedID, UserID);
    }
}

public long addUser(User user) {
    // TODO Check if the user already exists!!

    ContentValues temp = new ContentValues();

    temp.put(USER_COLUMN_USERNAME, user.getUserName());
    temp.put(USER_COLUMN_USERID, user.getId());
    temp.put(USER_COLUMN_IMGURL, user.getProfileImageURL());
    // TODO implement source on user?

    long userID = db.insert(TABLE_USER, null, temp);
    return userID;
}

public void addFeedUserBridge(long feedID, long userID) {

    ContentValues temp = new ContentValues();

    temp.put(FEEDUSER_COLUMN_FEED_ID, feedID);
    temp.put(FEEDUSER_COLUMN_USER_ID, userID);

    db.insert(TABLE_FEEDUSER, null, temp);
}

public void removeUserFromFeed(Feed feed, User user) {
    long feedID = getFeedID(feed);
    long userID = getUserID(user);

    removeFeedUserBridge(feedID, userID);
}

private void removeUser(User user) {

    long id = user.getId();
    db.delete(TABLE_USER, USER_COLUMN_ID + "=?", new String[] { id + "" });
}

private void removeFeedUserBridge(long feedID, long userID) {

    db.delete(TABLE_FEEDUSER, FEEDUSER_COLUMN_FEED_ID + "=?" + " and "
            + FEEDUSER_COLUMN_USER_ID + "=?", new String[] { feedID + "",
            userID + "" });
}

public ArrayList<String> listFeedUser() {
    final ArrayList<String> feeduser = new ArrayList<String>();

    Cursor c = db.rawQuery("SELECT * FROM " + TABLE_FEEDUSER, null);
    while (c.moveToNext()) {
        feeduser.add(c.getString(0) + " " + c.getString(1));
    }
    c.close();
    return feeduser;
}

private void removeFeedBridge(Long id) {

    db.delete(TABLE_FEEDUSER, FEEDUSER_COLUMN_FEED_ID + "=?",
            new String[] { id + "" });
}

public void updateUser(long userID) {
    // TODO implement this method
}

public void addListOfItems(List<? extends Item> itemList) {

    db.beginTransaction();
    for (Item i : itemList) {

        ContentValues temp = new ContentValues();
        temp.put(ITEM_COLUMN_TEXT, i.getText());
        temp.put(ITEM_COLUMN_TIMESTAMP, i.getTimestamp());
        temp.put(ITEM_COLUMN_TYPE, i.getText());
        temp.put(ITEM_COLUMN_URL, i.getURL());
        temp.put(ITEM_COLUMN_IMGURL, i.getIMGURL());
        temp.put(ITEM_COLUMN_USER_ID, i.getUser().getId());
        db.insert(TABLE_ITEM, null, temp);

    }
    db.setTransactionSuccessful();
    db.endTransaction();
    // db.close();
}

public Cursor getAllItems() {

    Cursor c = db.query(TABLE_ITEM, new String[] { ITEM_COLUMN_ID,
            ITEM_COLUMN_TEXT, ITEM_COLUMN_TIMESTAMP, ITEM_COLUMN_TYPE,
            ITEM_COLUMN_URL, ITEM_COLUMN_IMGURL, ITEM_COLUMN_USER_ID },
            null, null, null, null, null);
    // db.close();
    return c;
}

public long getItemTableSize() {
    SQLiteDatabase database = this.getReadableDatabase();
    long l = DatabaseUtils.queryNumEntries(database, TABLE_ITEM);
    // database.close();
    return l;
}

public void clearItemTable() {
    SQLiteDatabase db = getWritableDatabase();
    db.delete(TABLE_ITEM, null, null);
}

public void addUsers(List<? extends User> users) {

    db.beginTransaction();

    for (User u : users) {
        ContentValues temp = new ContentValues();
        temp.put(USER_COLUMN_USERNAME, u.getUserName());
        db.insert(TABLE_USER, null, temp);
    }
    db.setTransactionSuccessful();
    db.endTransaction();
}

public Cursor getAllUsers() {

    Cursor c = db.query(TABLE_USER, new String[] { USER_COLUMN_ID,
            USER_COLUMN_USERNAME }, null, null, null, null, null);
    return c;
}

public Cursor getUsers(Feed feed) {

    return null;
}

}

I’ve tried to search the web for information on how to “dynamic” queries, but havn’t found anything. :/

My guess is it would be something like:
Cursor c = db.rawQuery(“SELECT * FROM Item WHERE userID = (some argument to select all users from FeedUser table)”);

Anyone knows any helpful links on how to make such queries?

Best regards Rawa

Answer 1

You could use a query like this:

SELECT *
FROM Item
WHERE userID IN (SELECT userID
                 FROM FeedUser
                 WHERE feedID = ?)

But a join would be more efficient:

SELECT Item.field1, Item.field2, ...
FROM Item JOIN FeedUser
  ON Item.userID = FeedUser.userID