t, MySQL - rextester

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DROP TABLE IF EXISTS Users1;
DROP TABLE IF EXISTS Conversations1;
DROP TABLE IF EXISTS Messages1;
DROP TABLE IF EXISTS UserConversations1;

CREATE TABLE Users1
    (`userId` int, `username` varchar(6))
;
    
INSERT INTO Users1
    (`userId`, `username`)
VALUES
    (87, 'aester'),
    (89, 'cester'),
    (96, 'bester')
;

CREATE TABLE Conversations1
    (`conversationId` int)
;
    
INSERT INTO Conversations1
    (`conversationId`)
VALUES
    (67),
    (68)
;

CREATE TABLE Messages1
    (`messageId` int, `text` varchar(5), `timestamp` varchar(8), `conversation_id` int, `user_id` int)
;
    
INSERT INTO Messages1
    (`messageId`, `text`, `timestamp`, `conversation_id`, `user_id`)
VALUES
    (41, 'Hello', '00:00:00', 67, 87),
    (42, 'Hey', '00:00:00', 68, 89)
;

CREATE TABLE UserConversations1
    (`id` int, `conversation_id` int, `user_id` int)
;
    
INSERT INTO UserConversations1
    (`id`, `conversation_id`, `user_id`)
VALUES
    (3, 67, 87),
    (4, 67, 96),
    (5, 68, 89)
;

SELECT uc.`conversation_id`,
       GROUP_CONCAT(u.`username`)  as participants      
FROM UserConversations1 uc
JOIN Users1 u
  ON uc.`user_id`= u.`userId` 
GROUP BY `conversation_id`  
HAVING COUNT(IF(uc.`user_id` = 87, 1, NULL)) > 0;

SELECT `conversation_id` , MAX(`messageId`) as `messageId`
FROM Messages1
GROUP BY `conversation_id`;

SELECT conversation.`conversation_id`,
       conversation.`participants`,
       m.text,
       m.timestamp
FROM (  SELECT uc.`conversation_id`,
               GROUP_CONCAT(u.`username`)  as `participants`      
        FROM UserConversations1 uc
        JOIN Users1 u
          ON uc.`user_id`= u.`userId` 
        GROUP BY `conversation_id`  
        HAVING COUNT(IF(uc.`user_id` = 87, 1, NULL)) > 0) as conversation
JOIN ( SELECT `conversation_id` , MAX(`messageId`) as `messageId`
       FROM Messages1
       GROUP BY `conversation_id`) as last_message
  ON conversation.`conversation_id` = last_message.`conversation_id` 
JOIN Messages1 m
  ON m.`messageId` = last_message.`messageId`;


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