->expression('chid',DatabaseConnection::prefixTables('COALESCE((SELECT chid FROM {poll_choices} c WHERE {poll_votes}.chorder = c.chorder AND {poll_votes}.nid = c.nid), 0)'))
->expression('chid',Database::getConnection()->prefixTables('COALESCE((SELECT chid FROM {poll_choices} c WHERE {poll_votes}.chorder = c.chorder AND {poll_votes}.nid = c.nid), 0)'))