How do I generate a random sample of groups, including all people in the group, where the group_id (but not the person_id) changes across time? - postgresql

Question

I have data that looks like this:
+----------+-----------+------------+------+
| group_id | person_id | is_primary | year |
+----------+-----------+------------+------+
| aaa1 | 1 | TRUE | 2000 |
| aaa2 | 1 | TRUE | 2001 |
| aaa3 | 1 | TRUE | 2002 |
| aaa4 | 1 | TRUE | 2003 |
| aaa5 | 1 | TRUE | 2004 |
| bbb1 | 2 | TRUE | 2000 |
| bbb2 | 2 | TRUE | 2001 |
| bbb3 | 2 | TRUE | 2002 |
| bbb1 | 3 | FALSE | 2000 |
| bbb2 | 3 | FALSE | 2001 |
+----------+-----------+------------+------+
The data design is such that
person_id uniquely identifies an individual across time
group_id uniquely identifies a group within each year, but may change from year to year
each group contains primary and non-primary individuals
My goal is three-fold:
Get a random sample, e.g. 10%, of primary individuals
Get the data on those primary individuals for all time periods they appear in the database
Get the data on any non-primary individuals that share a group with any of the primary individuals that were sampled in the first and second steps
I'm unsure where to start with this, since I need to first pull a random sample of primary individuals and get all observations for them. Presumably I can do this by generating a random number that's the same within any person_id, then sample based on that. Then, I need to get the list of group_id that contain any of those primary individuals, and pull all records associated with those group_id.
I don't know where to start with these queries and subqueries, and unfortunately, the interface I'm using to access this database can't link information across separate queries, so I can't pull a list of random person_id for primary individuals, then use that text file to filter group_id in a second query; I have to do it all in one query.

Answer 1

A quick way to get this done is:
select
data_result.*
from
data as data_groups join
(select
person_id
from
data
where
is_primary
group by
person_id
order by
random()
limit 1) as selected_primary
ON (data_groups.person_id = selected_primary.person_id)
JOIN data AS data_result ON (data_groups.group_id = data_result.group_id AND data_groups.year = data_result.year)
I even made a fiddle so you can test it.
The query is pretty straightforward, it gets the sample, then it gets their groups and then it gets all the users of those groups.
Please pay atention to the Limit 1 clause that is there because the data set was so little. You can put a value or a query that gets the right percentage.
If anyone has an answer using windowing functions I'd like to see that.
Note: next time please provide the schema and the data insertion so it is easier to answer.