You are sure you entered right database name, database user and server but still show error “Error establishing a database connection” on the front, what the reason ? If you met that in a multisite WordPress installation then it may caused by wrong domain name setting in you database.
1. Go to the dabhoboard such as http://localhost/wordpress-401-mu/wp-admin you will see the following screen.
2. As the screen printed it cannot find out the site localhost/wordpress-401-mu/ in your database table mur_blogs ( if you use default database prefix then it is wp_blogs ).
3. Follow the instruction and open you database with phpmyadmin( or other database management tool ), and open the table mur_blogs ( default wp_blogs ). Plase change the domain & path to correct as the follow.
* Because you use localhost as the domain in you XAMPP, so domain always should be localhost, and the primary site may be localhost/your-directory-name/sub_site/