Hur beräknar man maximala inkommande samtalstoppar?

När ett samtal startar ökar antalet samtal. När ett samtal avslutas minskar antalet samtal. Så...

;with cte as
(
    select SessionStartTime as changetime,1 as CC from yourtable
    union all
    select SessionCloseTime,-1 from yourtable
)
    select top 1 changetime,rt from
    (
    select * from cte
        cross apply 
        (select SUM(cc) as rt from cte c where c.changetime<=cte.changetime) rt         
    ) v
    order by rt desc