The sql language is a very well established standard for retrieving information from relational databases and has recently been enhanced with new facilities called " structured types " that support nested structures similar to the nesting of elements in xml Sql语言是一个用于从关系数据库中检索信息的稳定标准，并且在最近还增加了名为“结构化类型”的新工具，用以支持类似于xml中元素嵌套的嵌套结构。
