This answers your question, quoted from http://www.stata.com/help.cgi?missing

replace  d = 0 if wage == .                        
* replace d to 0 if wage equals missing value
summarize wage cndwage if wage < .            
* summarize wage and cndwage when the missing value of wage is excluded
replace wage0 = 0 if wage >= .
* replace wage0 to 0 if wage is missing
               
Quote :
*"        ., the default, which is called the "system missing value" or sysmiss


    and


        .a, .b, .c, ..., .z, which are called the "extended missing values".


    Numeric missing values are represented by large positive values.  The
    ordering is


                all nonmissing numbers < . < .a < .b < ... < .z


    Thus, the expression age > 60 is true if variable age is greater than 60
    or missing.


    To exclude missing values, ask whether the value is less than ".".  For
    instance,


        . list if age > 60 & age < .


    To specify missing values, ask whether the value is greater than or equal
    to ".".  For instance,


        . list if age >=.


    Stata has one string missing value, which is denoted by "" (blank).  "*

再问下这个大于号、小于号一般怎么区分使用呢？

大于和小于号就按正常的理解来区分，  >大于,<小于,>=大于或者等于,<=小于或者等于

在这里就把缺失值当做一个特例，一个无限足够大的数集，一个正常的数据所不能达到的水平，只有大于等于这个临界(.) 才是缺失值，小于就是正常观测值。

原来这样啊，差不多明白了，stata里的表示有点奇怪，谢谢了

这些就需要看manual里面介绍stata对缺失值的介绍和解释了