比如，在国有企业的大类别下，一共有多少个企业。也就是说有多少企业属于国有企业，有多少企业属于外商独资企业等这样。这里的code是企业编码，就是中外合资企业里有多少不同的企业编码，这个命令该怎么写呢？
* Example generated by -dataex-. To install: ssc install dataex
clear
input str18 nature double code
"国有企业"       3105915011
"中外合资企业" 2102230397
"中外合资企业" 4202930018
"中外合资企业" 3702935175
"中外合资企业" 4202930018
"中外合资企业" 4202930018
"中外合资企业" 4202930018
"中外合资企业" 2102230338
"中外合资企业" 4202930018
"中外合资企业" 4202930018
"中外合资企业" 4202930018
"中外合资企业" 2102230338
"国有企业"       1102919062
"外商独资企业" 3205240073
"外商独资企业" 3205240073
"外商独资企业" 3205240073
"外商独资企业" 3205240073
"外商独资企业" 3205240352
"外商独资企业" 3205240073
"外商独资企业" 3122441906
"外商独资企业" 3122441906
"外商独资企业" 3122441906
"外商独资企业" 3122441906
"外商独资企业" 3122441906
"外商独资企业" 3122441906
"外商独资企业" 3122441906
"外商独资企业" 3205240073
"外商独资企业" 3122441906
"外商独资企业" 3205240352
"外商独资企业" 3122441906
"外商独资企业" 3205240352
"外商独资企业" 3205240073
"外商独资企业" 3205240073
"外商独资企业" 3205240073
"外商独资企业" 3122441906
"外商独资企业" 3122441906
"外商独资企业" 3205240073
"外商独资企业" 3205240073
"外商独资企业" 3205240073
"外商独资企业" 3205240073
"外商独资企业" 3122441906
"外商独资企业" 3205240073
"外商独资企业" 3122441906
"私营企业"       4420960515
"私营企业"       2106960419
"私营企业"       4420960515
"私营企业"       4419960100
"私营企业"       4420960515
"中外合资企业" 4423930528
"中外合资企业" 1213935087
"中外合资企业" 3205230076
"中外合资企业" 3205230076
"中外合资企业" 4423930234
"中外合资企业" 4423930234
"中外合资企业" 3205230076
"中外合资企业" 4423930234
"中外合资企业" 4423930234
"中外合资企业" 4423930528
"中外合资企业" 4423930528
"中外合资企业" 2102230397
"中外合资企业" 4423930528
"中外合资企业" 2102230338
"中外合资企业" 1213935087
"中外合资企业" 3205230076
"中外合资企业" 2101933583
"中外合资企业" 4420932808
"中外合资企业" 6101330064
"中外合资企业" 4420932419
"中外合资企业" 4422931192
"中外合资企业" 6101330064
"中外合资企业" 4420932808
"中外合资企业" 4422930664
"中外合资企业" 4413933063
"中外合资企业" 4423930528
"中外合资企业" 1213935087
"中外合资企业" 4413933063
"中外合资企业" 3205230076
"中外合资企业" 4422931192
"中外合资企业" 1213935087
"中外合资企业" 4422931192
"中外合资企业" 4422931192
"中外合资企业" 4422931192
"中外合资企业" 4422931192
"中外合资企业" 1213935087
"中外合资企业" 4422931192
"中外合资企业" 4422931192
"中外合资企业" 2102230397
"中外合资企业" 4422930664
"中外合资企业" 4422930948
"中外合资企业" 1213935087
"中外合资企业" 4422931192
"中外合资企业" 4420932419
"中外合资企业" 4413933063
"中外合资企业" 4420932419
"中外合资企业" 2102230397
"中外合资企业" 4420932419
"中外合资企业" 3122230573
"中外合资企业" 3122230573
"中外合资企业" 4422931192
"中外合资企业" 4420932419
end