As join operation to relational database, structural join order selection is at the heart of query optimization in an XML database.
和关系数据库中的连接运算一样，结构连接顺序的选择是XML数据库查询优化的核心。
Another operation used in conjunction with relational database is the JOIN operation. It is used to combine different relations into one relation.
另一个联同关系数据库的操作时联接操作，联接操作被用来合并不同的关系为一个关系。
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