If you're a web-hosting company or a web-development using WordPress to build multitudes of customer blogs or just an independent blogger or sys-admin with a task to optimize a server's MySQL allocated storage / performance on triads of WordPress-es a a good tip that would help is to removing wp_comments marked as spam.
Even though sites might be protected of thousands of spam message daily caught by WP anti-spam plugin Akismet, spam caught messages aer forwarder by Akismet to WP's Spam filter and kept wp_comments table with comments_approved column record 'spam'.
Therefore you will certainly gain of freeing disk space uselessly allocated by spam messages into current MySQL server storage dir (/var/lib/mysql /usr/local/mysql/data – the directory where my.cnf tells the server to keep its binary data .MYI, .MYD, .frm files) as well as save a lot of disk space by excluding the useless spam messages from SQL daily backup archives.
Here is how to remove manually spam comments from a WordPress blog under database (wp_blog1);
mysql> use wp_blog1;
mysql> describe wp_comments;
| Field | Type | Null | Key | Default | Extra |
| comment_ID | bigint(20) unsigned | NO | PRI | NULL | auto_increment |
| comment_post_ID | bigint(20) unsigned | NO | MUL | 0 | |
| comment_author | tinytext | NO | | NULL | |
| comment_author_email | varchar(100) | NO | | | |
| comment_author_url | varchar(200) | NO | | | |
| comment_author_IP | varchar(100) | NO | | | |
| comment_date | datetime | NO | | 0000-00-00 00:00:00 | |
| comment_date_gmt | datetime | NO | MUL | 0000-00-00 00:00:00 | |
| comment_content | text | NO | | NULL | |
| comment_karma | int(11) | NO | | 0 | |
| comment_approved | varchar(20) | NO | MUL | 1 | |
| comment_agent | varchar(255) | NO | | | |
| comment_type | varchar(20) | NO | | | |
| comment_parent | bigint(20) unsigned | NO | MUL | 0 | |
| user_id | bigint(20) unsigned | NO | | 0 | |
The most common and quick way useful for scripting (whether you have to do it for multiple blogs with separate dbs) is to delete all comments being filled as 'Spam'.
To delete all messages which were filled by Akismet's spam filter with high probabily being a spam issue from mysql cli interface:
DELETE FROM wp_comments WHERE comment_approved = 'spam';
For Unread (Unapproved) messages the value of comment_approved field are 0 or 1, 0 if the comment is Red and Approved and 1 if still it is to be marked as read (and not spam).
If a wordpress gets heavily hammered with mainly spam and the probability that unapproved message is different from spam is low and you want to delete any message waiting for approvel as not being spam from wordpress use following SQL query:
DELETE FROM wp_comments WHERE comment_approved = 0;
Another not very common you might want to do is delete only all apprved comments:
DELETE FROM wp_comments WHERE comment_approved = 1;
For old installed long time unmaintained blogs (with garbish content), it is very likely that 99% of the messages might be spam and in case if there are already >= 100 000 spam messages and you don't have the time to inspect 100 000 spam comments to get only some 1000 legitimate and you want to delete completely all wordpress comments for a blog in one SQL query use:
Another scenario if you know a blog has been maintained until certain date and comments were inspected and then it was left unmaintained for few years without any spam detect and clear plugin like Akismet, its worthy to delete all comments starting from the date wordpress site stopped to be maintained:
DELETE FROM wp_comments WHERE comment_date > '2008-11-20 05:00:10' AND comment_date <= '2014-11-24 00:30:00'