SQL sequent grouped rows

Question

+------+------+------+------------+
| id   | room | type | created_at |
+------+------+------+------------+
| 8214*|   83 | msg* | 1571726466 |
| 8213 |   83 | msg* | 1571724983 |
| 8212 |   83 | ad   | 1571724982 |
| 8211 |   83 | msg  | 1571724978 |
| 8210 |   83 | msg  | 1570861659 |
| 8209 |   83 | msg  | 1570861656 |
| 8208 |   83 | msg  | 1570861650 |
| 8207 |   83 | ad   | 1570861643 |
| 8206 |   83 | msg  | 1570861632 |
| 8255*|   82 | msg* | 1571732235 |
| 8254 |   82 | msg* | 1571732235 |
| 8253 |   82 | msg* | 1571732235 |
| 8252 |   82 | msg* | 1571732235 |
| 8251 |   82 | msg* | 1571732234 |
| 8250 |   82 | ad   | 1571732234 |
| 8249 |   82 | msg  | 1571732234 |
| 8248 |   82 | msg  | 1571732234 |
| 8247 |   82 | msg  | 1571732234 |
| 8246 |   82 | msg  | 1571732234 |
| 8245 |   82 | msg  | 1571732233 |
| 8244 |   82 | msg  | 1571732233 |
| 8243 |   82 | msg  | 1571732233 |
| 8242 |   82 | ad   | 1571732232 |
| 8241 |   82 | msg  | 1571732232 |
| 8240 |   82 | msg  | 1571732232 |
| 8239 |   82 | msg  | 1571732231 |
| 8238 |   82 | msg  | 1571732231 |
| 8237 |   82 | ad   | 1571732231 |
| 8236 |   82 | msg  | 1571732231 |
| 8235 |   82 | msg  | 1571732231 |
| 8234 |   82 | msg  | 1571732230 |
| 8233 |   82 | msg  | 1571732230 |
| 8232 |   82 | msg  | 1571732230 |
| 8231 |   82 | msg  | 1571732230 |
| 8230 |   82 | msg  | 1571732230 |
| 8229 |   82 | msg  | 1571732229 |
| 8228 |   82 | msg  | 1571732228 |
| 8227 |   82 | msg  | 1571732228 |
| 8226 |   82 | msg  | 1571732228 |
| 8225 |   82 | msg  | 1571732227 |
| 8224 |   82 | msg  | 1571732227 |
| 8223 |   82 | ad   | 1571732227 |
| 8222 |   82 | msg  | 1571732226 |
| 8221 |   82 | msg  | 1571732223 |
| 8220 |   82 | msg  | 1571732223 |

I have table of 'messages'. There could be regular message or ad. Each message belongs to room. I need to get last messages by date in each room and in this selection must be count of each regular messages (type=msg) before first encounter of type 'ad' given that messages' order are descending.

Expected will probably explain better what I need:

+------+------+------+------------+-----------+
| id   | room | type | created_at | count_msg |
+------+------+------+------------+-----------+
| 8214 |   83 | msg  | 1571726466 | 2         |
| 8255 |   82 | msg  | 1571732235 | 5         |
+------+------+------+------------+-----------+

In the source I marked with '*' which rows should count/showup in the result.

PS. Sorry if grammar is wrong.

UPD: In case of same created_at I guess it should order by id desc as well.

Answer 1

From your expected output it seems you need all the msg rows after the last ad occurs. This an be easily achieved using a sub-query -

SELECT MAX(T1.id), T1.room, type, MAX(created_at), COUNT(*) count_msg
FROM YOUR_TABLE T1
JOIN (SELECT MAX(id) id, room
      FROM YOUR_TAB
      WHERE type = 'ad'
      GROUP BY room) T2 ON T1.id > T2.id AND T1.room = T2.room
GROUP BY room, type

Here is the fiddle.

Answer 2

You seem to want all messages after the last 'ad' -- at least based on your data.

I think about filtering using a correlated subquery and aggregation:

select t2.room, max(t.id), t.type, max(t.created_at), count(*)
from t
where t.id > (select max(t2.id)
              from t t2
              where t2.room = t.room and t2.type = 'ad'
             )
group by t.room, t.type;