influxdb 语法求助

[mrco]

下面是 influxdb 里已有的数据，（ VALUE 是可变的）：

> select * from exec_wanip
name: exec_wanip
time                host    value
----                ----    -----
1542856250000000000 monitor 118.114.187.208
1542856850000000000 monitor 118.114.187.208
1542857450000000000 monitor 118.114.187.208
1542858050000000000 monitor 118.114.187.208
1542858650000000000 monitor 118.114.187.208
1542859250000000000 monitor 118.114.187.208
1542859850000000000 monitor 118.114.187.208
1542860450000000000 monitor 118.114.187.208
1542861050000000000 monitor 118.114.187.208
1542861650000000000 monitor 118.114.187.208
1542862250000000000 monitor 118.114.187.208
1542862850000000000 monitor 118.114.187.208
1542863450000000000 monitor 118.114.187.208
1542864050000000000 monitor 118.114.187.208
1542864650000000000 monitor 118.114.187.208
1542865250000000000 monitor 118.114.187.208

需要计算出在某个时间段内，value 的持续时间（ last - first 的时间戳，再说直白点，就是计算 ip 地址存在的时长）。谢谢！！