warning是由于LENGTH和SET产生的？楼主的例子应该不会产生warning吧。

默认情况应该没有warning，是不是修改了什么选项？

1    data aaa;
2    input x $20.;
3    cards;

NOTE: The data set WORK.AAA has 1 observations and 1 variables.
NOTE: DATA statement used (Total process time):
      real time           0.10 seconds
      cpu time            0.04 seconds


5    ;
6    run;
7
8
9    data aaa;
10   length x $10.;
11   set aaa;
12   run;

NOTE: There were 1 observations read from the data set WORK.AAA.
NOTE: The data set WORK.AAA has 1 observations and 1 variables.
NOTE: DATA statement used (Total process time):
      real time           0.02 seconds
      cpu time            0.01 secon

proc options;
run;
Check varlenchk setup, default is warn, nowarn turn warning off.

proc sql;
    alter table aaa
    modify x char(10);
quit;

there is no warning, even if varlenchk=warn.