'Executes a SELECT query on the named table, which returns a list
comprising of the values of the named column from the selected rows.
Args:
table (str): table name
keyvalues (dict): column names and values to select the rows with
retcol (str): column whos value we wish to retrieve.
Returns:
Deferred: Results in a list'

def _simple_select_onecol(self, table, keyvalues, retcol, desc='_simple_select_onecol'):

    return self.runInteraction(desc, self._simple_select_onecol_txn, table, keyvalues, retcol)
