The SAS data sets Work.Employee and Work.Salary are shown below；


Work.Employee                                fname ageBruce 30Dan 40
Work.Salaryfname salaryBruce 25000Bruce 35000Dan 25000

The following merged SAS data set is generated:Work.Empdatafname age totsalBruce 30 60000Dan 40 25000
Which one of the following SAS programs created the merged data set?正解：data work.empdata(drop=salary);merge work.employeework.salary;by fname;if first.fname then totsal=0;totsal+salary;if last.fname then output;run;
不明白这个程序在计算机中是怎么运行出来的，是先把emplyee和salary合并么？然后再从第一条观测开始判断是first还是last？最后把salary删除了？