An example of the "relational" sort of thing I would want to use it for is a historical work database for my singing gigs (to help me remember what I've done!). I would be listing jobs, singers, conductors, venues, dates of performances, works performed etc. Any given singer might have performed in any number of jobs with me, any work might have been performed in any number of jobs, any job might have many works performed and even more than one conductor, and a work would have a composer and might have other properties such as translation or edition.
You can do that in SQLNote. Items can have multiple parents, items can be in different "grids", etc. Wikis aren't as powerful and aren't as flexible.
Like Pierre likes to say : Think of SQLNotes and its grids as a 3d container (like a cube) to allow you to have different perspectives on your data. Each face is a different way at looking at your own data. (Of course, the "container" could have as many faces as you want... a dodecahedron?...

)
SQLNotes can be used to organize many many things. SQLNotes can also export to html (you can also create special templates...).
Of course, even if I'm not yet an "expert", I could help you too... but this week is going to be pretty busy for me... I'll try.
In the mean time, you could start with the basic Wiki. Have a look the
Getting started page. In the Wiki documentation you'll learn a a bit about Items, Fields, Equations, Search/Filtering, etc.
Don't forget to also have a look at some of the examples! (look at the column on the left of the Getting started page) And look inside all the forums for help.

(IMO, the hardest think with SQLNotes might be all the subtleties allowing to display your data exactly the way you want (flat view, hierarchical view, save item state, filter criterias apply to subitems, etc.), and also the filters/sources "language" if you're not used to writing your own filters.)
PS : I,m also including the new alias I use with farr to search for info concerning SQLNotes! If you use farr, put the alias in your find and run robot alias directory.