Comment system for 2 different entities - mysql

Question

I am trying to create a comment system.
The database design for the things, I would like to comment on (posts and articles):
TABLE `posts` (
`post_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`post_text` text NOT NULL,
`user_id` int(11) unsigned NOT NULL,
PRIMARY KEY (`post_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
TABLE `articles` (
`article_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`article_text` text NOT NULL,
`user_id` int(11) unsigned NOT NULL,
PRIMARY KEY (`article_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
The problem
Each comment should be linked to either a post or an article.
My attempts
Option No.1
I put the possibility for an article_id as well as a post_id into the same table and they both can be left empty.
TABLE `comments` (
`comment_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`comment_text` text NOT NULL,
`user_id` int(11) unsigned NOT NULL,
`post_id` int(11) NULL,
`article_id` int(11) NULL,
PRIMARY KEY (`article_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
Like this there would be an article_id, when an article is commented and a post_id if a post is commented.
But is it a good idea to leave both fields NULL, if one should always be NOT NULL?
Option No.2
Creating two seperate tables. One for post comments and one for article comments.
TABLE `article_comments` (
`comment_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`comment_text` text NOT NULL,
`user_id` int(11) unsigned NOT NULL,
`article_id` int(11) NOT NULL,
PRIMARY KEY (`article_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
TABLE `post_comments` (
`comment_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`comment_text` text NOT NULL,
`user_id` int(11) unsigned NOT NULL,
`post_id` int(11) NOT NULL,
PRIMARY KEY (`article_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
Like this nobody, who should be NOT NULL is NULL.
However, I do not know if this might become a perfomance issue and I am pretty sure this would lead to a lot of repetition in my PHP stuff.
Is there a better way to do this?
I have no experience in such things and would be very thankful for help!

Answer 1

There are a few considerations here.
Object vs. Relational mappings
PHP offers OO-style programming which would allow you to create an abstract super-class of article and post, to which comments could then generically be linked. OO structures like this do not map well to relational databases though unfortunately and so you have to make compromises.
Use cases for accessing the data
How and where do you need to access the comment data. This can drive the way you structure and potentially de-normalise your data and create indexes also.
Where to enforce data integrity constraints
It is nice to be able to have the database enforce clean data when you can, but this isn't always possible in reality (or you have to jump through hoops like in your second option above) to achieve it. Personally, this is one of those cases where I wouldn't bother trying to get the database to manage it for me, but rather ensure this is managed (and error handled) appropriately within your application.
Ultimately, I have not come across one correct answer to this problem. Of the two options you suggest above, Option 1 would be my own preference. However, an alternative you should consider is to merge your Posts and Articles tables into one 'Content' table with a content_type property or similar (which allows you to model them with an abstract superclass and concrete sub-classes in PHP). Then your comments table just references that generically.