先上例子
data a;
input No;
datalines;
1
2
3
;
run;

data b;
input gg;
datalines;
1
;
run;

data all;
if _n_=1 then set b;
set a;
run;
proc print;run;

结果

all

ObsggNo
111
212
313

去掉if _n_=1 then 的话结果就是只有第一行，第一行我能理解，两个set把列合并了，取小的行数；但是不删去输出结果的机制实在是不明白，看了看一本书上的定义：Two automatic variables are created by every DATA step: _N_ and _ERROR_.
_N_
is initially set to 1. Each time the DATA step loops past the DATA statement, the
variable _N_ increments by 1. The value of _N_ represents the number of times
the DATA step has iterated.
还是不能理解这代码在干什么。求大神指导！谢谢~