MySQL 8.0

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CREATE TABLE chat_schema (`msg_id` INTEGER, `msg` VARCHAR(10), `from_user` INTEGER, `to_user` INTEGER);

INSERT INTO chat_schema (`msg_id`, `msg`, `from_user`, `to_user`) VALUES
  ('1', 'Hello!', '16', '77'),
  ('2', 'Wassup?', '16', '77'),
  ('3', 'Hey there!', '77', '16'),
  ('4', 'Hola!', '7', '77');
WITH cte AS (
  SELECT *, 
         ROW_NUMBER() OVER (
           PARTITION BY CASE WHEN from_user = 77 THEN to_user ELSE from_user END 
           ORDER BY msg_id DESC
         ) rn
  FROM chat_schema
  WHERE 77 IN (from_user, to_user)
)
SELECT msg_id, msg, from_user, to_user
FROM cte 
WHERE rn = 1
ORDER BY msg_id DESC;


msg_id
msg
from_user
to_user


4
Hola!
7
77

3
Hey there!
77
16


SELECT cs1.*
FROM chat_schema cs1
WHERE 77 IN (cs1.from_user, cs1.to_user)
  AND NOT EXISTS (
            SELECT *
            FROM chat_schema cs2
            WHERE (cs2.from_user, cs2.to_user) IN ((cs1.from_user, cs1.to_user), (cs1.to_user, cs1.from_user))
              AND cs2.msg_id > cs1.msg_id
          )
ORDER BY cs1.msg_id DESC;


msg_id
msg
from_user
to_user


4
Hola!
7
77

3
Hey there!
77
16