The problem is usually not that, but there are indeed cases where some people want to block “part of a site”.
E.g. some site that offers games (that are seen as unwanted, e.g. timewasting) and useful content.
In general it is better to use different methods than blocking to prevent the timewasting…
Well, when there is https and nondescript URLs (like ?param=abss%aasf&aasssf), then there isn’t much you can do, except offer warnings or incentives to spend time in better ways.
In this case, you can combine URL filter + DNS filter, to get best of both worlds, at least for http.
General problem of URL filtering for https still applies, though.
What I mean is that you probably should not try to solve it by blocking the unwanted content, but by explaining to your users
what content they are not supposed to visit and what will be the corrective actions when they still do it.
I call this layer 8 policy filtering. 
“Stop going to YouTube on company time or you’re fired.”