Dear Stata Users,

Below I have a sample of two datasets: set1 and set2. I need to merge them. The problem is that there are repetitive firms (gvkey) and date (trading_date) observation. So, I merge using many-to-many option. However, I came across this:

"Because m:m merges are such a bad idea, we are not going to show you an example. If you think that you need an m:m merge, then you probably need to work with your data so that you can use a 1:m or m:1 merge." (append-merge-collapse.pdf (arizona.edu)).

And I am curious if it is ok to merge m:m?


Set1

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input double gvkey float trading_date
10031 10350
10031 10349
10031 10346
10031 10345
10031 10344
10031 10343
10031 10342
10031 10339
10031 10338
10031 10337
10031 10336
10031 10335
10031 10332
10031 10331
10031 10330
10031 10329
10031 10328
10031 10325
10031 10324
10031 10323
10031 10322
10031 10321
10031 10317
10031 10316
10032 10350
10032 10349
10032 10346
10032 10345
10032 10344
10032 10343
10032 10342
10032 10339
10032 10338
10032 10337
10032 10336
10032 10335
10032 10332
10032 10331
10032 10330
10032 10329
10032 10328
10032 10325
10032 10324
10032 10323
10032 10322
10032 10321
10032 10317
10032 10316
end
format %td trading_date





Set2

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input double personid str2 status double gvkey float trading_date
12000001 "S" 10031 10324
12000001 "P" 10031 10330
12500004 "S" 10031 10330
12500004 "S" 10032 10324
12500005 "S" 10032 10324
12500005 "P" 10032 10324
12500005 "S" 10032 10330
12500010 "P" 10032 10330
12500010 "S" 10032 10330
end