I am trying to learn the basics of databases, but I am finding the terminology especially impenetrable compared to other things I have learned. Most of the database tutorials and documentation I have looked at explain everything in terms of other database terminology. Most examples only show code but not actual output, so I am often unclear as to what the expected output is. I am usually even less clear on what the intended use case is.
For example, take the term "scalar function" which I recently come across. It is defined here similarly to other definitions I found.
Scalar Function – Either a built-in SQL function or a user-defined function that returns a single value computed only from the values of any required arguments at the time the function is called.
Why is that special? How is that different from just a function that outputs one value? Why is the fact that the arguments are "required arguments" a noteworthy distinction? Why, in practical terms, would it be important to specify that a scalar function is to be used?
I would appreciate both an answer to this specific question and references to database (preferably Postgres) learning materials that make a point to explain each technical term in the least technical way possible.