Structured Query Language (SQL), is a declarative language invented to perform database operations. Its primary qualities are the ability to express what you want, rather than how you want it ("I want a set of items that conform to a predicate X" versus "Filter every item using predicate X"); this also allows non-programmers to work with databases, which is an aspect that today's NoSQL databases often lack.
You may think: how is that relevant? SQL allows us to think of the data as sets linked together with relations, which is what makes it so pleasant to work with. Understanding sets as a distinct collection of objects is sufficient to understand the language and how to manipulate the results. While this definition is also called the naive set theory, it is a useful definition for most purposes.
In general, a set has elements as members that can be...