I followed instruction as mentioned in Wordpress, but I still got the error and referred Google and I was shocked to see many blog have same problem and they are not fixed yet and those blogs are indexed in Google, please try searching "Call to undefined function require_wp_db()" or find here
Any advice how to fix? Thanks in advance
This will occasionally occur if you do not upload wp-config.php or all of the other files that have changed between versions.
Download a clean copy of the WP version you are trying to upgrade from the WP Release Archive.
Update wp-config.php with your database details, secret keys, etc.
Delete all files except for /wp-content on your server (hopefully you have kept all of your custom enhancements relegated to the /wp-content/themes directory as they recommend - otherwise you will have to diff against each individual file unless you've kept good track of your changes).
Upload all of the clean files except for the /wp-content directory
Run /wp-admin/upgrade.php