最后的结果，出现了“type mismatch" 的错误。。。不知道是怎么回事。。

You probably attempted to generate a new string variable and
        forgot to specify its type, so Stata assumed the new variable
        was to be numeric.  Include a str# before the name of the new
        variable in your generate statement.  More generally, in an
        expression you attempted to combine a string and numeric
        subexpression in a logically impossible way.  For instance,
        you attempted to subtract a string from a number or you
        attempted to take the substring of a number.

如果可以的话，你加我QQ:519904247 我把具体的数据给你发过去。这个问题真的是很急。今天下午5点的时候就得考试。。在这里先谢谢了。。。

Most likely, your "id" is a string variable, try this first:

encode id, gen (id_n)

then change the last command as follows:

collapse (sum) export (min) id_min=id_n (max) id_max=id_n, by (group)

The following is a better one to convert the string "id" variable:

destring id, gen (id_n1)

then change the last command as follows:

collapse (sum) export (min) id_min=id_n1 (max) id_max=id_n1, by(group)

刚才有试了一下。出现了r(111) error . . . . . . . . . . . . . . . . . . . . . . . .  Return code 111
        __________ not found;
        no variables defined;
        The variable does not exist.  You may have mistyped the
        variable's name.
        variables out of order;
        You specified a varlist containing varname1-varname2, yet
        varname1 occurs after varname2.  Reverse the order of the
        variables if you did not make some other typographical error.
        Remember, varname1-varname2 is taken by Stata to mean varname1,
        varname2, and all the variables in dataset order in between.
        Type describe to see the order of the variables in your dataset.
        __________ not found in using data;
        You specified a varlist with merge, yet the variables on which
        you wish to merge are not found in the using dataset, so the
        merge is not possible.
        __________ ambiguous abbreviation;
        You typed an ambiguous abbreviation for a variable in your data.
        The abbreviation could refer to more than one variable.  Use a
        nonambiguous abbreviation or, if you intend all the variables
        implied by the ambiguous abbreviation, append a `*' to the end
        of the abbreviation.

学写了！