譬如一個 banner 可以有多種 theme，一個 theme 可以屬於多種 banner，banner 和 theme 之間的關係及屬於多對多。

以此紀錄設計多對多關係的 tables，並利用 JOIN 取得關聯資料。

CREATE TABLE banners (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  image_url VARCHAR(50),
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
);

CREATE TABLE themes (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  color VARCHAR(50),
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

CREATE TABLE banner_theme (
  banner_id INT,
  theme_id IINT,
  PRIMARY KEY (banner_id, theme_id),
  FOREIGN KEY (banner_id) REFERENCES banners(id) ON DELETE CASCADE,
  FOREIGN KEY (theme_id) REFERENCES themes(id) ON DELETE CASCADE,
);

INSERT INTO banners (id, name, image_url) VALUES (1, 'Banner 1', 'http://example.com/banner1.jpg');
INSERT INTO banners (id, name, image_url) VALUES (2, 'Banner 2', 'http://example.com/banner2.jpg');
INSERT INTO themes (id, name, color) VALUES (1, 'Theme 1', 'red');
INSERT INTO themes (id, name, color) VALUES (2, 'Theme 2', 'blue');
INSERT INTO banner_theme (banner_id, theme_id) VALUES (1, 1);
INSERT INTO banner_theme (banner_id, theme_id) VALUES (1, 2);
INSERT INTO banner_theme (banner_id, theme_id) VALUES (2, 2);

SELECT banners.name, themes.name, themes.color
FROM banners
JOIN banner_theme ON banners.id = banner_theme.banner_id
JOIN themes ON banner_theme.theme_id = themes.id;