我看文献上说_N_代表读取数据集的条数,很模糊的意思,不是很懂,请教高人解答一下。data shoes; infile 'c:\tlsb\shoesales.dat'; input name $ 1-15 type $ sales;run;proc means noprint data=shoes; var sales; output out =summarydata sum(sales)=total;run;proc print data=summarydata;run;data summaryshoes; if _n_=1 then set summarydata; set shoes; percent=sales / total * 100;run;proc print data=summaryshoes; var name type sales total percent;run;我知道黄色是代表将两个数据集合并,但不知道什么原理,希望讲解一下。