Mysql TimeStampDiff最小时间减去最大时间 - 分组错误

Question

目标：计算座席每天在每个广告系列中花费的总时间。 问题：使用最小值和最大值结束＆＃34;不正确＆＃34;结果。 当前查询：

SELECT agent,campaign,DATE(eventfield) AS created,TIMESTAMPDIFF(SECOND,MIN(TIMESTAMP(eventfield)),MAX(TIMESTAMP(eventfield))) / 3600 AS Hours
FROM log1
WHERE eventfield > '2015-01-19'
AND eventfield < '2015-01-20'
GROUP BY agent,campaign,created

示例数据：

agent    campaign    eventfield
alfalfa  apples      2015-01-19 09:00:00
alfalfa  apples      2015-01-19 09:11:00
alfalfa  pears       2015-01-19 09:11:00
alfalfa  pears       2015-01-19 16:30:00
alfalfa  apples      2015-01-19 16:30:00
alfalfa  apples      2015-01-19 17:00:00

预期结果：

agent    campaign    created      hours
alfalfa  apples      2015-01-19   0.6833
alfalfa  pears       2015-01-19   7.3167

实际结果：

agent    campaign    created      hours
alfalfa  apples      2015-01-19   8.0000
alfalfa  pears       2015-01-19   7.3167