Set theory is the underlying concept of SQL. A set is a collection of zero or more objects. Each object in a set is called an element. In MySQL, a table corresponds to a set, and a record corresponds with an element. You can get a subset from a set. A subset is a smaller set of elements from the set. In SQL, you can get a subset by using a WHERE clause. A cross product is a set created from two or more sets. In SQL, a cross product is a join. To create different sets of data in SQL, you can use the intersection, difference, and union joins. Each of these is explained in more detail in the following sections.