You can inquire about wikipedia
Basically, it's a bit like Excel for storing data. Well, I say a little bit. Because we can store data on sql but we can't do calculations. But on the other hand to store data, it's great. Because we can choose according to the data, the space dedicated to this data. We can also do certain types of operations. Data processing between them by comparing two tables for example.
I do not know if I have been clear enough. In any case, for the messages that we write on a site like a forum or social network, bah, what we write is stored on a sql language. There are different versions. Me again, I use the most rudimentary trick. But it's huge. It's enough for me at my level.
I propose you this gallery of images.
I did not have any particular problem. It's not that obvious. You have to be careful what you do. I would even say very careful according to the importance of the base of data. Well, I'm a little rusty on this language, it's been a while since I used it.