mysql - 为什么在列上设置键约束会从中删除 "NOT NULL"(MySQL)？

Question

我有创建表格的代码:

DROP TABLE IF EXISTS `database`.`creatures_kinds` ;

CREATE TABLE IF NOT EXISTS `database`.`creatures_kinds` (
  `id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
  `parent_id` INT UNSIGNED NULL,
  `name_en` VARCHAR(255) NOT NULL,
  PRIMARY KEY (`id`, `parent_id`),
  INDEX `parent_id` (`parent_id` ASC),
  CONSTRAINT `parent`
    FOREIGN KEY (`parent_id`)
    REFERENCES `database`.`creatures_kinds` (`id`)
    ON DELETE RESTRICT
    ON UPDATE CASCADE)
ENGINE = InnoDB;

令我惊讶的是，新创建的表如下所示:

为什么 parent_id设置为NOT NULL当我写NULL时在 CREATE TABLE ？

这和PRIMARY KEY有关系吗？或FOREIGN KEY ？

我猜parent_id是一个可识别的关系(它指向更一般的生物种类，例如从 pigeon 到 bird ，并且它是 NULL 代表顶级生物种类)。

Answer 1

A PRIMARY KEY is a unique index where all key columns must be defined as NOT NULL. If they are not explicitly declared as NOT NULL, MySQL declares them so implicitly (and silently). MySQL Manual