To delete all pending comments in WordPress, you can use the following MySQL statement:
DELETE FROM wp_comments WHERE comment_approved = '0';
This statement will delete all comments that have a comment_approved value of '0', which means they are pending.
Before running this statement, make sure to replace "wp_" with the correct prefix for your WordPress database tables. The default prefix is "wp_", but it can be different if you have changed it during the WordPress installation process.
It is also a good idea to back up your database before running any delete statements, in case you need to restore the data later.
Here is an example of how you can run this statement using the MySQL command-line client:
mysql> USE database_name;
mysql> DELETE FROM wp_comments WHERE comment_approved = '0';
Replace "database_name" with the name of your WordPress database.
You can also run this statement using a tool like phpMyAdmin or by using a WordPress plugin that allows you to run custom SQL queries.