SAS help错了？关于set/merge +by语句中，pdv变量置为缺失的情况的讨论-经管之家官网！

在诸如此类语句时。
data three;
set one two;
by var;
run;
SAS help中说明：
The values of the variables in　the program data vector are set to missing each time SAS starts to read a new
data set and when the BY group changes.　（SASlanguage reference 9.2, P362）
但我经过测试，我发现其实除了首次将ＰＤＶ置为缺失时，ＳＡＳ开始读另一个data set和by 组改变时，均没有置为缺失。
以下是我的测试：

1. data one;
   
2. x=1;output;
   
3. x=1;output;
   
4. x=3 ;output;
   
5. run;
   
6. 
   
7. data two;
   
8. x=2;output;
   
9. x=2;output;
   
10. x=4;output;
    
11. run;
    
12. 
    
13. proc sort data =one;
    
14. by x;
    
15. run;
    
16. proc sort data=two;
    
17. by x;
    
18. 
    
19. data three;
    
20. put "before set:" _all_;
    
21. set one two;
    
22. by x;
    
23. put "after set:" _all_;
    
24. run;

复制代码log:148data three;
149put "before set:" _all_;
150 set one two;
151 by x;
152put "after set:" _all_;
153run;
before set:x=. FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=1
after set:x=1 FIRST.x=1 LAST.x=0 _ERROR_=0 _N_=1
before set:x=1 FIRST.x=1 LAST.x=0 _ERROR_=0 _N_=2
after set:x=1 FIRST.x=0 LAST.x=1 _ERROR_=0 _N_=2
before set:x=1 FIRST.x=0 LAST.x=1 _ERROR_=0 _N_=3
after set:x=2 FIRST.x=1 LAST.x=0 _ERROR_=0 _N_=3
before set:x=2 FIRST.x=1 LAST.x=0 _ERROR_=0 _N_=4
after set:x=2 FIRST.x=0 LAST.x=1 _ERROR_=0 _N_=4
before set:x=2 FIRST.x=0 LAST.x=1 _ERROR_=0 _N_=5
after set:x=3 FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=5
before set:x=3 FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=6
after set:x=4 FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=6
before set:x=4 FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=7
NOTE: There were 3 observations read from the data set WORK.ONE.
NOTE: There were 3 observations read from the data set WORK.TWO.
NOTE: The data set WORK.THREE has 6 observations and 1 variables.
NOTE: DATA statement used (Total process time):
real time 0.01 seconds
cpu time 0.01 seconds
标红部分都是by组改变，且要切换到另一个data set里读取观测。但是pdv里的值是retain的。没有置为missing.
同样，对于ＭＥＲＧＥ+by语句：
SAS HELP说明：
When SAS has read all observations in a
BY group from all data sets, it sets all variables in the program data vector
(except those created by SAS) to missing　（SASlanguage reference 9.2, P373）

1. data a;
   
2. x=1; y='a1';output;
   
3. x=2;y='a2';output;
   
4. x=3 ;y='a3';output;
   
5. run;
   
6. 
   
7. data b;
   
8. x=1;z='b1';output;
   
9. x=2;z='b2';output;
   
10. x=3;z='b3';output;
    
11. run;
    
12. 
    
13. proc sort data=one;
    
14. by x;
    
15. run;
    
16. proc sort data=two;
    
17. by x;
    
18. run;
    
19. 
    
20. data c;
    
21. put "before set:" _all_;
    
22. merge a b;
    
23. by x;
    
24. put "after set:" _all_;
    
25. run;

复制代码ＬＯＧ:
198data c;
199put "before set:" _all_;
200 merge a b;
201 by x;
202put "after set:" _all_;
203run;
before set:x=. y=z=FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=1
after set:x=1 y=a1 z=b1 FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=1
before set:x=1 y=a1 z=b1 FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=2
after set:x=2 y=a2 z=b2 FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=2
before set:x=2 y=a2 z=b2 FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=3
after set:x=3 y=a3 z=b3 FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=3
before set:x=3 y=a3 z=b3 FIRST.x=1 LAST.x=1 _ERROR_=0 _N_=4
NOTE: There were 3 observations read from the data set WORK.A.
NOTE: There were 3 observations read from the data set WORK.B.
NOTE: The data set WORK.C has 3 observations and 3 variables.
NOTE: DATA statement used (Total process time):
real time 0.01 seconds
cpu time 0.01 seconds　如上，标红部分也是by组change时，结果还是retain，没有置为Ｍissing.