MySQL 5.7

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CREATE TABLE users (
  `id` INTEGER,
  `username` VARCHAR(5),
  `avatar` VARCHAR(10)
);

INSERT INTO users
  (`id`, `username`, `avatar`)
VALUES
  ('1', 'user1', 'image1.png'),
  ('2', 'user2', 'image2.png');
CREATE TABLE projects (
  `id` INTEGER,
  `title` VARCHAR(8),
  `user_id` INTEGER
);

INSERT INTO projects
  (`id`, `title`, `user_id`)
VALUES
  ('1', 'project1', '1'),
  ('2', 'project2', '1');
CREATE TABLE images (
  `id` INTEGER,
  `image_name` VARCHAR(15),
  `project_id` INTEGER
);

INSERT INTO images
  (`id`, `image_name`, `project_id`)
VALUES
  ('1', 'attachment1.png', '1'),
  ('2', 'attachment2.png', '1');
    SELECT 
        p.id,
        p.title,
        u.username,
        u.avatar,
        i.image_name
    FROM
        users u INNER JOIN 
        projects p 
        ON u.id = p.user_id
        LEFT JOIN 
         images   i ON p.id = i.project_id and i.id = (SELECT MIN(`id`) FROM images WHERE project_id = i.project_id)
    WHERE
       u.id = 1


id
title
username
avatar
image_name


1
project1
user1
image1.png
attachment1.png

2
project2
user1
image1.png
null

id	title	username	avatar	image_name
1	project1	user1	image1.png	attachment1.png
2	project2	user1	image1.png	null