EXISTS • WITH
In Chapter 4, we talked about composite functions: functions that contained other functions. Similarly, SQL queries can contain other queries. Queries contained within other queries are called subqueries.
The topic of subqueries is somewhat complex, primarily because there are many different ways in which they can be used. Subqueries can be found in many different parts of the SELECT statement, each with different nuances and requirements. As a query contained within another query, a subquery can be related to and dependent on the main query, or it can be completely independent of the main query. Again, this distinction results in different requirements for their usage.
No matter how subqueries ...