不好意思，我也是新来的

* Data step;

data wanted(keep=mean_tcost);
set test end=last;
if _n_=1 then sumtcost=tcost;
else sumtcost+tcost;
if last then do;
  mean_tcost=sumtcost/_n_;
  output;
end;
run;

* Sql;

proc sql;
create table wanted as
select mean(tcost) as mean_tcost
from test;
quit;

啥都不说了，必须赞，万分感谢！