Here’s the SQL to reset the mediawiki password of a user:

use mediawikidb;
update tbl_user set user_password=md5(concat(user_id,'-',md5('newpassword'))) where user_name = "Alex";

The default admin username is WikiSysop, with user_id=1, so you could do:

update tbl_user set user_password=md5(concat('1-',md5('newadminpassword'))) where user_id=1;

Here’s how you add a new user:

insert into user(user_name) values ("Alex");

then set a password as above. Keep in mind that usernames must start with a capital letter.