MySQL 8.0

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CREATE TABLE tbl_user (
  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);

CREATE TABLE tbl_store (
  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  store VARCHAR(255) NOT NULL
);

CREATE TABLE tbl_user_store (
  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  fk_user INT NOT NULL,
  fk_store INT NOT NULL,
  FOREIGN KEY (fk_user) REFERENCES tbl_user(id),
  FOREIGN KEY (fk_store) REFERENCES tbl_store(id)
);

CREATE TABLE tbl_article (
  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  article_name VARCHAR(255) NOT NULL
);

CREATE TABLE tbl_user_entries (
  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  article_name VARCHAR(255) DEFAULT NULL,

fk_user INT NOT NULL,
  fk_article INT,
  fk_store INT NOT NULL,
  FOREIGN KEY (fk_user) REFERENCES tbl_user(id),
  FOREIGN KEY (fk_article) REFERENCES tbl_article(id),
  FOREIGN KEY (fk_store) REFERENCES tbl_store(id)
);

INSERT INTO tbl_user (name) VALUES ('Tom'), ('Cendric');
INSERT INTO tbl_store (store) VALUES ('Zurich'), ('Bern'), ('Luzern');
INSERT INTO tbl_user_store (fk_user, fk_store) VALUES (1, 1), (1, 2), (1, 3), (2, 1), (2, 3);
INSERT INTO tbl_article (article_name) VALUES ('article1'), ('article2'), ('article3');
INSERT INTO tbl_user_entries (article_name, fk_user, fk_article, fk_store) VALUES ('article4', 1, NULL, 1), (NULL, 1, 1, 3), (NULL, 1, 2, 2);

fk_user	fk_store	name	store	coalesce(ue.article_name, a.article_name)
1	1	Tom	Zurich	article4
1	3	Tom	Luzern	article1