Chapter 12. Recursive Queries
One of the major criticisms of SQL, up through and including SQL-92, was its inability to implement recursive processing. Many important problems that are difficult to solve by other means yield readily to recursive solutions. Extensions included in SQL:1999 allow recursive queries — which greatly expand the language's power. If your SQL implementation includes the recursion extensions, you can efficiently solve a large new class of problems. However, because recursion is not a part of core SQL, many implementations currently available do not include it.
What Is Recursion?
Recursion is a feature that's been around for years in programming languages such as Logo, LISP, and C++. In these languages, you can define a function (a set of one or more commands) that performs a specific operation. The main program invokes the function by issuing a command called a function call. If the function calls itself as a part of its operation, you have the simplest form of recursion.
A simple program that uses recursion in one of its functions provides an illustration of the joys and pitfalls of recursion. The following program, written in C++, draws a spiral on the computer screen. It assumes that the drawing tool is initially pointing toward the top of the screen, and includes three functions:
n) draws a line n units long.
d) rotates the drawing tool d degrees counterclockwise.
You can define the function
spiral(segment)as follows: ...