data three;
set one;
put _all_;
output;
set two;
put _all_;
/*output;*/
run;
proc print ;
run;

The results will be as follows:


Obs    x    y            z

  1     1    Groucho
  2     3    Harpo      Chico
  3     3    Kart       Zeppo

data three;
put _all_;
set one;
put _all_;
/*output;*/
set two;
put _all_;
output;  /*This is always an implicit statement when there is no same statement ahead*/
run;
proc print ;
run;

the resulsts will be as follows:

Obs    x       y         z

1     2    Groucho    Chico
2     4    Harpo      Zeppo

For data set two, there exist no obs corresponding to _N_=3.  the data loop will leave out here. so the results will be only 2 obs.

By default, every DATA step contains an implicit OUTPUT statement at the end of each iteration that tells SAS to write observations to the data set or data sets that are being created.

Placing an explicit OUTPUT statement in a DATA step overrides the automatic output, and SAS adds an observation to a data set only when an explicit OUTPUT statement is executed.

Once you use an OUTPUT statement to write an observation to any one data set, however, there is no implicit OUTPUT statement at the end of the DATA step. In this situation, a DATA step writes an observation to a data set only when an explicit OUTPUT executes.

You can use the OUTPUT statement alone or as part of an IF-THEN or SELECT statement or in DO-loop processing.

进来学习下

sas给每个set分配一个指针,指针读到文件的结尾(EOF)时,退出Data步.同理,以下的结果Result中,其观测数为a,b,c,d中最小的那个观测数.

Something about where a DATA Step starts and ends:

1. A DATA Step always starts from the "Data" statement

2. A DATA Step stops at:

推荐你看一看姚志勇《SAS编程与数据挖掘商业案例》，写得很明白

I have a better understanding about the output statement now. Thank you!

Thank you very much~~~The data step is very interesting~

好~最近看的是朱世武那本……

根据sas certificate prep guide 第十三章 combining sas data set  one-to-onereading 里面有这样一句话  the data step stops after it has read the last observation from the smallest data set.但是通过log的确从数据集 WORK.ONE. 读取了 3 个观测。难道书不正确？怎么理解？

真心看不懂。。。