代码

select
hour(click_time) as click_hour
,count(hour(click_time)) as click_cnt
from user_ad_click_time
group by click_hour
order by click_cnt desc
limit 1

知识点

order by和limit的连用，取出所需结果

YEAR() 返回统计的年份
MONTH() 返回统计的月份
DAY() 返回统计的天
HOUR() 返回小时值
MINUTE() 返回分钟值
SECOND() 返回秒数
WEEK() 全年第几周