使用ContentProvider和CursorLoader从Android中的SQLiteDatabase获取Distinct值

Question

我正在尝试使用contentprovider查询和CursorLoader从我的数据库中检索不同的值。虽然CursorLoader不允许不同的规范，但我发现了setDistinct方法，可以将其添加到查询构建器中以添加此规范。我没有找到想要的结果，并且好奇为什么。我的查询如下所示

@Override
    public Cursor query(Uri uri, String[] projection,
                        String selection, String[] selectionArgs, String sortOrder) {

    // create SQLiteQueryBuilder for querying flower table
    SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();
    queryBuilder.setTables(Flower.TABLE_NAME);
    queryBuilder.setDistinct(true);

    switch (uriMatcher.match(uri)) {
        case OneFlower: // contact with specified id will be selected
            queryBuilder.appendWhere(
                    Flower._ID + "=" + uri.getLastPathSegment());
            break;
        case CONTACTS: // all contacts will be selected
            break;
        default:
            throw new UnsupportedOperationException(
                    getContext().getString(R.string.invalid_query_uri) + uri);
    }

    Cursor cursor = queryBuilder.query(dbHelper.getReadableDatabase(),
            projection, selection, selectionArgs, null, null, sortOrder);

    // configure to watch for content changes
    cursor.setNotificationUri(getContext().getContentResolver(), uri);
    return cursor;
} 

而我的CursorLoader看起来像这样

@Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        switch (id) {
            case LOADER_ID:
                return new CursorLoader(
                        getContext(),
                        DatabaseDescription.Flower.CONTENT_URI,
                        FROM_COLUMNS,
                        COLUMN_LOCATION + "<> ''",
                        null,
                        COLUMN_LOCATION + " ASC"
                );
            default:
                if (BuildConfig.DEBUG)
                    throw new IllegalArgumentException("no id handled!");
                return null;
        }
    }