当DATA步里用BY语句时，就会对每一个BY语句中的变量产生一个 first.var 和一个last.var。

当某一条观测是该BY-变量某个组（值相同）的第一条时，first.var的值是1，否则为0;
当某条观测是该BY-变量某个组的最后一条时，则last.var的值是1，否则为0.

if last.id; 相当于 if last.id=1;  所以只会输出每个组的最后一条。


288  data temp;
289  merge a b;
290  by id;
291  put _all_;
292  if last.id;
293  run;


id=1 score=100 name=nagato FIRST.id=1 LAST.id=0 _ERROR_=0 _N_=1
id=1 score=200 name=nagato FIRST.id=0 LAST.id=0 _ERROR_=0 _N_=2
id=1 score=300 name=nagato FIRST.id=0 LAST.id=1 _ERROR_=0 _N_=3
id=2 score=100 name=sakura FIRST.id=1 LAST.id=0 _ERROR_=0 _N_=4
id=2 score=200 name=sakura FIRST.id=0 LAST.id=0 _ERROR_=0 _N_=5
id=2 score=300 name=sakura FIRST.id=0 LAST.id=1 _ERROR_=0 _N_=6


NOTE: There were 6 observations read from the data set WORK.A.
NOTE: There were 2 observations read from the data set WORK.B.
NOTE: The data set WORK.TEMP has 2 observations and 3 variables.
NOTE: DATA statement used (Total process time):
      real time           0.09 seconds
      cpu time            0.03 seconds