selecting min date by 2 grouping variables

[Sigrid Dirgis]

My dataset has 2 grouping variables: ID and drug name
Also date for starting the treatment with a certain drug but per one drug can be several dates or the date is missing, I need to select the earliest date within a drug.
Data looks like this:

patient_id	start_date	drug
1562	07.01.2019	drug1
1562	15.04.2019	drug1
1562	15.04.2019	drug1
1562	15.04.2019	drug1
1562	18.03.2020	drug1
1562	.	drug1
1562	18.03.2020	drug1
1562	08.11.2022	drug2
1562	08.11.2022	drug2
1562	21.02.2023	drug3

I know command bysort id : egen mindate = min(start_date)
But this does not work for me because one person may have multiple starting dates when starting with a new drug. I would be very grateful if anyone could help.

[Nick Cox]

bysort id drug (start_date): gen min_date_drug = start_date[1]

egen is sometimes needed for better treatment of missing values, and so forth, but here that is unlikely to bite.

[Sigrid Dirgis]

I got the error: weights not allowed

[alejoforero]

You probably included a space in the code between start_date and the square brackets, which stata interprets as the option for weights. The code as Nick wrote runs well.

[Sigrid Dirgis]

Yes, I had a space and now it works. Thank you very much to both of you