How to select rows based on properties of another row? - postgresql

Question

Had a question..
| a_id | name | r_id | message | date
_____________________________________________
| 1 | bob | 77 | bob here | 1-jan
| 1 | bob | 77 | bob here again | 2-jan
| 2 | jack | 77 | jack here. | 2-jan
| 1 | bob | 79 | in another room| 3-feb
| 3 | gill | 79 | gill here | 4-feb
These are basically accounts (a_id) chatting inside different rooms (r_id)
I'm trying to find the last chat message for every room that jack a_id = 2 is chatting in.
What i've tried so far is using distinct on (r_id) ... ORDER BY r_id, date DESC.
But this incorrectly gives me the last message in every room instead of only giving the last message in everyroom that jack belongs to.
| 2 | jack | 77 | jack here. | 2-jan
| 3 | gill | 79 | gill here | 4-feb
Is this a partition problem instead distinct on?

Answer 1

I would suggest :
to group the rows by r_id with a GROUP BY clause
to select only the groups where a_id = 2 is included with a HAVING clause which aggregates the a_id of each group : HAVING array_agg(a_id) #> array[2]
to select the latest message of each selected group by aggregating its rows in an array with ORDER BY date DESC and selecting the first element of the array : (array_agg(t.*))[1]
to convert the selected rows into a json object and then displaying the expected result by using the json_populate_record function
The full query is :
SELECT (json_populate_record(null :: my_table, (array_agg(to_json(t.*)))[1])).*
FROM my_table AS t
GROUP BY r_id
HAVING array_agg(a_id) #> array[2]
and the result is :
a_id
name
r_id
message
date
1
bob
77
bob here
2022-01-01
see dbfiddle

Answer 2

For last message in every chat room simply would be:
select a_id, name, r_id, to_char(max(date),'dd-mon') from chats
where a_id =2
group by r_id, a_id,name;
Fiddle https://www.db-fiddle.com/f/keCReoaXg2eScrhFetEq1b/0
Or seeing messages
with last_message as (
select a_id, name, r_id, to_char(max(date),'dd-mon') date from chats
where a_id =1
group by r_id, a_id,name
)
select l.*, c.message
from last_message l
join chats c on (c.a_id= l.a_id and l.r_id=c.r_id and l.date=to_char(c.date,'dd-mon'));
Fiddle https://www.db-fiddle.com/f/keCReoaXg2eScrhFetEq1b/1
Though all this complication could by avoided with a primary key on your table.