How select a row and count row in single query in postgresql? - sql

Question

I have a table in postgresql as follow:
id | chat_id | content | time | read_times
----+---------+-----------+------+-------------------------------------------------------------------------
1 | chat_10 | content_1 | t1 | [{"username": "user1", "time": 123}, {"username": "user2", "time": 111}]
2 | chat_10 | content_2 | t2 | [{"username": "user2", "time": 1235}]
3 | chat_10 | content_3 | t3 | []
4 | chat_11 | content_4 | t4 | [{"username": "user1", "time": 125}, {"username": "user3", "time": 121}]
5 | chat_11 | content_5 | t5 | [{"username": "user1", "time": 126}, {"username": "user3", "time": 127}]
Note: t1 < t2 < t3 < t4 < t5
After every user read a message, we registered it in read_times column(user2 read a message with id 2 at time 1235), Now I want to get user chat list with unread count chats. for user1 the result is as follow:
chat_id | content | unread_count
--------+-----------+--------------
chat_10 | content_3 | 2
chat_11 | content_5 | 0
Note: unread_count is messages count that user didn't read in a caht_id.
Is it possible with one query?

Answer 1

First, you must extract the user names for each chat_id and content with json_array_elements() function and with FIRST_VALUE() window function get the last content of each chat_id.
Then aggregate and combine SUM() window function with MAX() aggregate function to get the column unread_count:
WITH cte AS (
SELECT t.chat_id, t.content,
FIRST_VALUE(t.content) OVER (PARTITION BY t.chat_id ORDER BY t.time DESC) last_content,
(r->>'username') username
FROM tablename t LEFT JOIN json_array_elements(read_times::json) r ON true
)
SELECT DISTINCT c.chat_id, MAX(c.last_content) "content",
SUM((MAX((COALESCE(username, '') = 'user1')::int) = 0)::int) OVER (PARTITION BY c.chat_id) unread_count
FROM cte c
GROUP BY c.chat_id, c.content
ORDER BY c.chat_id
See the demo.