<?xml version="1.0" encoding="UTF-8"?>
<root>
<author>
<name>A</name>
<book>Book1</book>
<book>Book2</book>
</author>
<author>
<name>B</name>
<age>45</age>
<book>Book3</book>
</author>
</root>
如何编写XQuery以显示作者的书籍总数?找到子元素的总数?