Regarding "Index" in SQL, I searched on the Internet and understood the theoretical part that it is like searching in a library, that instead of searching the whole library for a specific book, by indexing, we can search for the book by its first letter (i.e; Books that begin with the letter "A"), which in this case makes the search much faster.
But, I didn't understand the technical point here.
Say I create an index like this:
What is the preceding line trying to tell me? Maybe the answer will be that I want to search by "coindex", but what is really confusing me is that WHEN will I use indexing. I'm not making any search, so, WHERE can I feel "index" in my SQL command.