喔，call symputx('rc','1');在where语句前面，只要没有数据读入的话，这句就直接跳过不执行吗，有些诡异，谢谢！

这是where语句efficiency的地方
For each iteration of the DATA step, the first operation SAS performs in each execution of a SET, MERGE, MODIFY, or UPDATE statement is to determine whether the observation in the input data set meets the condition of the WHERE statement. The WHERE statement takes effect immediately after the input data set options are applied and before any other statement in the DATA step is executed. If a DATA step combines observations using a WHERE statement with a MERGE, MODIFY, or UPDATE statement, SAS selects observations from each input data set before it combines them.

试一试下面的程序
data test1;
        a = 1;
run;
data test2;
        set test1;
        a = 2;
        where a = 1;
run;
你会发现test2有1行数据，且a=2。说明在读入test1的时候先判断a是否等于1，等于1就读入，然后执行赋值语句a = 2。
同理执行
data test3;
        set test1;
        a = 2;
        where a = 2;
run;
你会发现test3没有数据输出，因为在读入test1的时候没有一条数据满足a = 2。

明白了，多谢！！！刚用下面的程序试了下，貌似当没有数据读入时，就直接结束data步了，其它所有的语句都不执行了
proc contents data=sashelp.class out=_name_(keep=name) noprint;
run;

data _null_;
  set _name_;
  put "wrong";
  where upcase(name)="AGE2";   
  put "right";
  b=3;
  put "just test";
  put b=;
run;