When does WordPress write to the database?

Question

MySQL

In what cases does WordPress write something to the database? The obvious seems to be understandable when someone leaves a comment there, or when someone posts a post, or when someone logs in, but when no one logs in, no one writes anything, but just people read the site - there are some database entries?
And just another question. Is there any way to cache between php and mysql? So that php does not pull the database every time, but takes the information from somewhere closer, from some kind of cache?

Answer 1

a just people reading the site - are there any records in the database?

yes, sessions are written/updated, at least.
There are thousands of them: memcache, radish, x-cache, apc, shared memory in the end)
There is a W3Cache plugin for WordPress, quite functional, it allows you to choose any popular caching method. From my experience, I can say that the memcache driver is disgusting there. I use apc on two projects, there seem to be no complaints.