MySQL 8.0

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CREATE TABLE IF NOT EXISTS user (
  `id` CHAR(40) CHARACTER SET 'utf8' COLLATE 'utf8_bin' NOT NULL,
  PRIMARY KEY (`id`)
);

CREATE TABLE IF NOT EXISTS `tag` (
  `id` INT(11) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(255) NULL DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE INDEX `name` (`name` ASC)
);

CREATE TABLE IF NOT EXISTS `bookmark` (
  `id` INT(11) NOT NULL AUTO_INCREMENT,
  `user_id` CHAR(36) CHARACTER SET 'utf8' COLLATE 'utf8_bin' NOT NULL,
  `link_id` INT(11) NOT NULL,
  PRIMARY KEY (`id`),
  INDEX `fk_bookmark_user1_idx` (`user_id` ASC),
  CONSTRAINT `fk_bookmark_user1`
    FOREIGN KEY (`user_id`)
    REFERENCES `user` (`id`)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION
);

CREATE TABLE IF NOT EXISTS `bookmark_tag` (
  `bookmark_id` INT(11) NOT NULL,
  `tag_id` INT(11) NOT NULL,
  PRIMARY KEY (`bookmark_id`, `tag_id`),
  INDEX `fk_bookmark_tag_bookmark1_idx` (`bookmark_id` ASC),
  INDEX `fk_bookmark_tag_tag1_idx` (`tag_id` ASC),
  CONSTRAINT `fk_bookmark_tag_bookmark1`
    FOREIGN KEY (`bookmark_id`)
    REFERENCES `bookmark` (`id`)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION,
  CONSTRAINT `fk_bookmark_tag_tag1`
    FOREIGN KEY (`tag_id`)
    REFERENCES `tag` (`id`)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION
);

INSERT INTO `user` 
  (`id`)
VALUES 
  ('e4e2bb46'),
  ('b95274c9');

INSERT INTO `tag` 
  (`id`, `name`) 
VALUES 
 (1, 'foo'),
 (2, 'bar'),
 (3, 'baz');

INSERT INTO `bookmark`
  (`id`, `user_id`, `link_id`)
VALUES
  (1, 'e4e2bb46', 1),
  (2, 'e4e2bb46', 2),
  (3, 'e4e2bb46', 3),
  (4, 'b95274c9', 4),
  (5, 'b95274c9', 5);
  
INSERT INTO `bookmark_tag`
  (`bookmark_id`, `tag_id`)
VALUES
	(1, 1),
	(1, 2),
	(2, 2),
	(2, 3),
	(3, 1),
	(4, 1),
	(4, 3);

SELECT 
   `user`.id,
    JSON_ARRAYAGG(
      JSON_OBJECT(
        "id", tag.id
      )
    ) AS tags,
    JSON_ARRAYAGG(
      JSON_OBJECT(
        "id", bookmark.id
      )
    ) AS bookmarks
  FROM tag
  INNER JOIN bookmark_tag ON bookmark_tag.tag_id = tag.id 
  INNER JOIN bookmark ON bookmark.id = bookmark_tag.bookmark_id
  INNER JOIN `user` ON user.id = bookmark.user_id
  GROUP BY `user`.id;

Records: 2  Duplicates: 0  Warnings: 0

Records: 3  Duplicates: 0  Warnings: 0

Records: 5  Duplicates: 0  Warnings: 0

Records: 7  Duplicates: 0  Warnings: 0

id	tags	bookmarks
b95274c9	[{"id": 1}, {"id": 3}]	[{"id": 4}, {"id": 4}]
e4e2bb46	[{"id": 1}, {"id": 2}, {"id": 2}, {"id": 3}, {"id": 1}]	[{"id": 1}, {"id": 1}, {"id": 2}, {"id": 2}, {"id": 3}]