How to obtain the highest daily traffic flow data that hour

hi! I want to get the highest daily traffic by day, so I try this as below

... | convert timeformat="%Y/%m/%d" ctime(_time) as Date | stats count as c by` Date,date_hour | sort 1 - c

But the num of sort can't use a variable

If I use this

... | convert timeformat="%Y/%m/%d" ctime(_time) as Date | stats count as c by Date,date_hour | stats max(c) as MAX by Date

I can't get the date_hour...

I want to get the result as..

Date  date_hour c
2013/8/13 15 100
2013/8/14 1  111
2013/8/15 6  91

Can someone help me find how to deal with it?

Thank you a lot. m(_ _)m