您可以将初始化移动到子查询中:

You can move the initialization into a subquery:

SELECT *
FROM (SELECT o.timestamp , o.v_phase_a , o.v_phase_b , o.v_phase_c , (@x := @x+1) AS x
      FROM operational_parameters AS o cross join
           (select @x := 0) const
      WHERE o.timestamp >= '2014-01-21 05:00:00' AND o.timestamp <= '2014-02-04 04:59:30'
      ORDER BY o.timestamp
     ) t
WHERE x MOD 336 = 0;