If you want to resolve this error there can be 2 ways:
1) Empty(Truncate) all the cache tables from your database. For example empty all tables suffix by "cache_" like cache, cache_block, cache_content, cache_filter, cache_form, cache_menu, cache_page, cache_update, cache_views.
2) Your MySQL credentials are not right. Check your settings.php see if your MySQL credentials are right. Either your database username, passport or host is wrong.