The built-in functions are a standard set, defined in the Functions and Operators recommendation and supported by all XQuery implementations. A detailed description of each built-in function is provided in Appendix A, and most are also discussed at appropriate places in the book.

The Functions and Operators recommendation defines built-in operators in addition to functions. These operators are like functions, but they cannot be called directly from a query; instead, they back up a symbol or keyword in the XQuery language. For example, the numeric-add operator backs up the + sign when both operands are numeric. Because they are internal to the implementation, and not used by the query author, they are not discussed directly in this book.

A user-defined function is one that is specified by a query author, either in the query itself, or in an external library. The second half of this chapter explains how to define your own functions in detail.