php - 使用 KEY/CONSTRAINT 与 FOREIGN KEY 之间的区别？

Question

我正在使用 Laravel 框架的迁移功能在数据库中创建外键。使用代码

Schema::table('posts', function($table){
    $table->integer('category_id')->unsigned();
    $table->foreign('category_id')
          ->references('id')
          ->on('categories');
});

生成 MySQL 输出(来自 SHOW CREATE TABLE 帖子)

`category_id` int(10) unsigned NOT NULL,
KEY `posts_category_id_foreign` (`category_id`),
CONSTRAINT `posts_category_id_foreign` FOREIGN KEY (`category_id`) REFERENCES `categories` (`id`)

为什么 Laravel 使用 KEY 和 CONSTRAINT ... FOREIGN KEY ... REFERENCES ... 来创建新索引而不是更常见的 FOREIGN KEY ... REFERENCES ... 在引用外键的列上？

Answer 1

因为 MySql 简单地忽略内联引用规范，所以他们在这里写到:http://dev.mysql.com/doc/refman/5.6/en/create-table.html

MySQL does not recognize or support “inline REFERENCES specifications” (as defined in the SQL standard) where the references are defined as part of the column specification. MySQL accepts REFERENCES clauses only when specified as part of a separate FOREIGN KEY specification.