能不能查查帮助啊

或者把基础的书先看看。

----------------------------------------------------------------------------------------------------------help for if                                                                manual:  [U] 14 Language syntax----------------------------------------------------------------------------------------------------------

if qualifier

        command if exp

    exp in the syntax diagram means an expression, such as age>21.  See help exp for an explanation of    expressions.

    There is another if used in Stata programming; see help ifcmd.

Description

    if at the end of a command means the command is to use only the data specified.  if is allowed with    most Stata commands.

Examples

    . list make mpg if mpg>25

    . list make mpg if mpg>25 & mpg25 | mpg