1. A streaming query system for extensible markup language, comprising: an XPath query translator receiving and analyzing a user-input XPath (Extensible Markup Language Path Language) document; an abstract syntax tree builder receiving a result obtained from the analyzing process of the XPath query translator, and building an abstract syntax tree accordingly; an XML parser receiving an Extensible Markup Language (XML) document, and parsing nodes of the XML document; an index generator generating an index of the XML document according to the result obtained by the analyzing process of the XML parser; and a computation module receiving the abstract syntax tree built by the abstract syntax tree builder, performing a format computation and obtaining a query result accordingly.