mysql滑动聚合

滑动聚合是按顺序对滑动窗口范围内的数据进行聚合的操作。下累积聚合不同，滑动聚合并不是统计开始计算的位置到当前位置的数据。

这里以统计最近三个月中员工第月订单情况为例来介绍滑动聚合。

滑动聚合和累积聚合解决方案的主要区别在于连接的条件不同。滑动聚合条件不再是b.ordermonth <= a.ordermonth,而应该是b.ordermonth大于前三个月的月份，并且小于当前月份。因此滑动聚合的解决方案的SQL语句如下

SELECT
  a.empid,
  DATE_FORMAT(a.ordermonth, '%Y-%m') AS ordermonth,
  a.qty AS thismonth,
  SUM(b.qty) AS total,
  CAST(AVG(b.qty) AS DECIMAL(5,2)) AS avg
FROM emporders a 
INNER JOIN emporders b
    ON a.empid=b.empid
    AND b.ordermonth > DATE_ADD(a.ordermonth, INTERVAL -3 MONTH)
    AND b.ordermonth <= a.ordermonth
WHERE DATE_FORMAT(a.ordermonth,'%Y')='2015' AND DATE_FORMAT(b.ordermonth,'%Y')='2015'
GROUP BY a.empid,DATE_FORMAT(a.ordermonth, '%Y-%m'),a.qty
ORDER BY a.empid,a.ordermonth

运行结果如下

该解决方案返回的是三个月为一个周期的滑动聚合，但是每个用户包含前两个月并且未满3个月的聚合。如果只希望返回满3个月的聚合，不返回未满3个月的聚合，可以使用HAVING过滤器进行过滤，过滤的条件为MIN(b.ordermonth)=DATE_ADD(a.ordermonth, INTERVAL -2 MONTH),例如

SELECT
  a.empid,
  a.ordermonth AS ordermonth,
  a.qty AS thismonth,
  SUM(b.qty) AS total,
  CAST(AVG(b.qty) AS DECIMAL(5,2)) AS avg
FROM emporders a 
INNER JOIN emporders b
    ON a.empid=b.empid
    AND b.ordermonth > DATE_ADD(a.ordermonth, INTERVAL -3 MONTH)
    AND b.ordermonth <= a.ordermonth
WHERE DATE_FORMAT(a.ordermonth,'%Y')='2015' AND DATE_FORMAT(b.ordermonth,'%Y')='2015' AND a.empid=1
GROUP BY a.empid,DATE_FORMAT(a.ordermonth, '%Y-%m'),a.qty
HAVING MIN(b.ordermonth)=DATE_ADD(a.ordermonth, INTERVAL-2 MONTH)
ORDER BY a.empid,a.ordermonth

运行结果如下