在sas advanced的prep guide的120页有这么一个in-line view：
select destination,
avg(delay) as average,
max(delay) as max,
sum(delay > 0) as late,
sum(delay <= 0) as early
from sasuser.flightdelays
group by destination
我搞不明白为什么
sum(delay <= 0) as early
这句会给出一个正数的early？如果sum一些<=0的数字不是应该是一个负数吗？