By a “set” we mean any collection M into a whole of definite, distinct objects m (which are called the “elements” of M) of our perception or of our thought.
—George Cantor, in “Georg Cantor” by Joseph W. Dauben (Princeton University Press, 1990)
Standard SQL is based on the relational model, which is a mathematical model for data management and manipulation. The relational model was initially created and proposed by Edgar F. Codd in 1969. Since then, it has been explained and developed by Chris Date, Hugh Darwen, and others.
Transact-SQL (T-SQL) is the main language used to manage and manipulate data in Microsoft SQL Server.
In fact, T-SQL is based more on multiset theory than on set theory. A multiset (also known as a bag or a superset) in many respects is similar to a set, but can have duplicates. The T-SQL language does give you enough tools so
that if you want to follow relational theory, you can do so. For example, the language provides you with a DISTINCT clause to remove duplicates.
It then moves on to describe one of the most important concepts you need to know about the language—logical query processing.