从基本节点派生出两个新类型的节点：一个用于视频，一个用于播客。
Two new types of nodes are derived from the base node: one for video and another for a podcast.
这行代码可以创建一个element类型的新节点，元素名为img。
This will create a new node, of type element, with the element name img. In HTML, that's essentially this.
XMLCAST：这个新的SQL函数将xml节点的值转换为关系数据类型，比如VARCHAR、INTEGER、DATE等等。
XMLCAST: a new SQL function that converts an XML node's value into a relational data type, such as VARCHAR, INTEGER, DATE, and so on.
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