SQL design for a navigation menu with one of two columns - mysql

Question

I have navigation menus which have links either to an external url, or to a local page identified by the pages PK pages_id.
To implement, I created the following tables.
pages stores the HTML for the center part in each page.
menus stores the name of the menu.
pages_has_menus adds the menu to the page, and specified whether it is a main menu, sub menu, etc as specified by menu_types.
menus_has_pages specifies the actual items in the menu.
How can I ensure that menus_has_pages doesn't contain both a pages_id and a external_url, and also always contains one or the other?
Should the schema be changed by adding addition tables for each scenario?
CREATE TABLE IF NOT EXISTS `pages` (
`id` INT UNSIGNED NOT NULL,
`sites_id` INT UNSIGNED NOT NULL,
`html` TEXT NOT NULL,
PRIMARY KEY (`id`),
INDEX `fk_pages_sites_idx` (`sites_id` ASC),
CONSTRAINT `fk_pages_sites`
FOREIGN KEY (`sites_id`)
REFERENCES `sites` (`id`)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;
CREATE TABLE IF NOT EXISTS `menu_types` (
`type` CHAR(4) NOT NULL,
PRIMARY KEY (`type`))
ENGINE = InnoDB;
CREATE TABLE IF NOT EXISTS `menus` (
`id` INT UNSIGNED NOT NULL,
`name` VARCHAR(45) NULL,
`sites_id` INT UNSIGNED NOT NULL,
PRIMARY KEY (`id`),
INDEX `fk_menus_sites1_idx` (`sites_id` ASC),
CONSTRAINT `fk_menus_sites1`
FOREIGN KEY (`sites_id`)
REFERENCES `sites` (`id`)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;
CREATE TABLE IF NOT EXISTS `pages_has_menus` (
`pages_id` INT UNSIGNED NOT NULL,
`menus_id` INT UNSIGNED NOT NULL,
`menu_types_type` CHAR(4) NOT NULL,
PRIMARY KEY (`pages_id`, `menus_id`),
INDEX `fk_pages_has_menus_menus1_idx` (`menus_id` ASC),
INDEX `fk_pages_has_menus_pages1_idx` (`pages_id` ASC),
INDEX `fk_pages_has_menus_menu_types1_idx` (`menu_types_type` ASC),
UNIQUE INDEX `unique_menu` (`pages_id` ASC, `menu_types_type` ASC),
CONSTRAINT `fk_pages_has_menus_pages1`
FOREIGN KEY (`pages_id`)
REFERENCES `pages` (`id`)
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_pages_has_menus_menus1`
FOREIGN KEY (`menus_id`)
REFERENCES `menus` (`id`)
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_pages_has_menus_menu_types1`
FOREIGN KEY (`menu_types_type`)
REFERENCES `menu_types` (`type`)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;
CREATE TABLE IF NOT EXISTS `menus_has_pages` (
`menus_id` INT UNSIGNED NOT NULL,
`pages_id` INT UNSIGNED NULL,
`external_url` VARCHAR(45) NULL,
`order` TINYINT NULL,
`name` VARCHAR(45) NULL,
`new_window` TINYINT NULL,
PRIMARY KEY (`menus_id`, `pages_id`),
INDEX `fk_menus_has_pages_pages1_idx` (`pages_id` ASC),
INDEX `fk_menus_has_pages_menus1_idx` (`menus_id` ASC),
CONSTRAINT `fk_menus_has_pages_menus1`
FOREIGN KEY (`menus_id`)
REFERENCES `menus` (`id`)
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `fk_menus_has_pages_pages1`
FOREIGN KEY (`pages_id`)
REFERENCES `pages` (`id`)
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;

Answer 1

I guess I would use two tables instead of yours:
one with:
pages_id INT UNSIGNED NOT NULL,
and one with :
external_url VARCHAR(45) NOT NULL,
And use 'UNION' when you need to merge two 'SELECT' queries.