外键约束Mysql

Question

I'm trying to create a relationship between a blog and an image, where a blog can have at most one image (think like avatar), but it can also have none. Therefore, I put the foreign key on the blogs table. I was trying to create a constraint on the images table where if the blog gets deleted, its associated image also gets deleted from the database. However, I'm not sure how to create the constraint when the foreign key is not in the table that should be cascading. Here's my tables, minus those fields that aren't relevant:

CREATE TABLE `blogs` (
  `id` int(10) unsigned NOT NULL auto_increment,
  `image_id` int(10) unsigned default NULL,
  PRIMARY KEY  (`id`),
) ENGINE=InnoDB AUTO_INCREMENT=921 DEFAULT CHARSET=utf8;

CREATE TABLE `images` (
  `id` int(10) unsigned NOT NULL auto_increment,
  PRIMARY KEY  (`id`),
) ENGINE=InnoDB AUTO_INCREMENT=251021 DEFAULT CHARSET=utf8 

Any help?

Answer 1

The only reason to have the images in a separate table would be if the same image could be shared by multiple blogs. In that case you would not want to delete on cascade anyway. You would need special logic to determine if there were no blogs left that referenced that image to delete it -- something that you could not do through DDL.

Instead, simply make the image values nullable columns on the blogs table.