Добавление объединения к запросу, вызывающему проблемы

Question

Мне дали задание по работе с приложением в чате. Там был список всех чатов пользователя. До сих пор у него было только 1 к 1 чату. Это была структура таблиц.

Чаты:

И сообщения

В настоящее время в нем были только одиночные чаты, поэтому chats.type ограничивался только одиночными.

Этот запрос я использовал

SELECT 
  * 
FROM 
  (
    SELECT 
      "" AS nickname, 
      chats.id AS chat_id, 
      chats.last_message_id, 
      chats.updated_at, 
      chats.type,
      Unix_timestamp(chats.updated_at) AS unix_time, 
      users.firstname, 
      users.lastname, 
      users.profile_image, 
      messages.message, 
      messages.is_read, 
      messages.sender_id, 
      chats.sender_id AS user_id, 
      (
        SELECT 
          Count(messages.id) 
        FROM 
          messages 
        WHERE 
          chats.id = messages.chat_id 
          AND receiver_id = 76 
          AND (is_read = 0 || is_read = 1)
      ) AS un_read_count 
    FROM 
      chats 
      JOIN users ON users.id = chats.sender_id 
      JOIN messages ON messages.id = chats.last_message_id 
    WHERE 
      chats.receiver_id = 76 
      AND chats.last_message_id != 3 
    UNION 
    SELECT 
      "" AS nickname, 
      chats.id AS chat_id, 
      chats.last_message_id, 
      chats.updated_at, 
      chats.type,
      Unix_timestamp(chats.updated_at) AS unix_time, 
      users.firstname, 
      users.lastname, 
      users.profile_image, 
      messages.message, 
      messages.is_read, 
      messages.sender_id, 
      chats.receiver_id AS user_id, 
      (
        SELECT 
          Count(messages.id) 
        FROM 
          messages 
        WHERE 
          chats.id = messages.chat_id 
          AND receiver_id = 76 
          AND (is_read = 0 || is_read = 1)
      ) AS un_read_count 
    FROM 
      chats 
      JOIN users ON users.id = chats.receiver_id 
      JOIN messages ON messages.id = chats.last_message_id 
    WHERE 
      chats.sender_id = 76 
      AND chats.last_message_id != 3
  ) AS r 
ORDER BY 
  r.updated_at

Сейчас Я интегрировал групповые чаты тоже. Я добавил третий UNION, но я не понимаю, что в нем не будет chats.sender_id или chats.receiver_id, так как можно получить эту информацию на основе chat_members.user_id, чтобы я мог также просматривать групповые чаты. Любая помощь или направление высоко ценится. Спасибо.

Это таблица chat_members

Это то, что я пробовал до сих пор

SELECT 
  * 
FROM 
  (
    SELECT 
      "" AS nickname, 
      chats.id AS chat_id, 
      chats.last_message_id, 
      chats.updated_at, 
      chats.type,
      Unix_timestamp(chats.updated_at) AS unix_time, 
      users.firstname, 
      users.lastname, 
      users.profile_image, 
      messages.message, 
      messages.is_read, 
      messages.sender_id, 
      chats.sender_id AS user_id, 
      (
        SELECT 
          Count(messages.id) 
        FROM 
          messages 
        WHERE 
          chats.id = messages.chat_id 
          AND receiver_id = 76 
          AND (is_read = 0 || is_read = 1)
      ) AS un_read_count 
    FROM 
      chats 
      JOIN users ON users.id = chats.sender_id 
      JOIN messages ON messages.id = chats.last_message_id 
    WHERE 
      chats.receiver_id = 76 
      AND chats.last_message_id != 3 
    UNION 
    SELECT 
      "" AS nickname, 
      chats.id AS chat_id, 
      chats.last_message_id, 
      chats.updated_at, 
      chats.type,
      Unix_timestamp(chats.updated_at) AS unix_time, 
      users.firstname, 
      users.lastname, 
      users.profile_image, 
      messages.message, 
      messages.is_read, 
      messages.sender_id, 
      chats.receiver_id AS user_id, 
      (
        SELECT 
          Count(messages.id) 
        FROM 
          messages 
        WHERE 
          chats.id = messages.chat_id 
          AND receiver_id = 76 
          AND (is_read = 0 || is_read = 1)
      ) AS un_read_count 
    FROM 
      chats 
      JOIN users ON users.id = chats.receiver_id 
      JOIN messages ON messages.id = chats.last_message_id 
    WHERE 
      chats.sender_id = 76 
      AND chats.last_message_id != 3
    UNION 
    SELECT 
      chats.group_name AS nickname, 
      chats.id AS chat_id, 
      chats.last_message_id, 
      chats.updated_at, 
      chats.type,
      Unix_timestamp(chats.updated_at) AS unix_time, 
      "" AS users.firstname, 
      "" AS users.lastname, 
      "" AS users.profile_image, 
      messages.message, 
      "" AS messages.is_read, 
      messages.sender_id,
      chats.receiver_id AS user_id, 
      (
        SELECT 
          Count(messages.id) 
        FROM 
          messages 
        WHERE 
          chats.id = messages.chat_id 
          AND receiver_id = 76 
          AND (is_read = 0 || is_read = 1)
      ) AS un_read_count 
    FROM 
      chats 
      JOIN users ON users.id = chats.receiver_id 
      JOIN messages ON messages.id = chats.last_message_id 
    WHERE 
      chats.sender_id = 76 
      AND chats.last_message_id != 3
      UNION 
    SELECT 
      chats.group_name AS nickname, 
      chats.id AS chat_id, 
      chats.last_message_id, 
      chats.updated_at, 
      Unix_timestamp(chats.updated_at) AS unix_time, 
      "" AS users.firstname, 
      "" AS users.lastname, 
      "" AS users.profile_image, 
      messages.message, 
      "" AS messages.is_read, 
      messages.sender_id,
      '' AS user_id, 
      '' AS un_read_count 
    FROM 
      chats 
      JOIN users ON users.id = chats.receiver_id 
      JOIN messages ON messages.id = chats.last_message_id 
    WHERE 
      chats.sender_id = 76 
      AND chats.last_message_id != 3
  ) AS r 
ORDER BY 
  r.updated_at