The derived relational algebra operators are widely used in the relational database query languages.

关系代数的派生算子在关系数据库查询语言中得到了广泛应用。

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Based on this model, this paper constructs two different kinds of medium relational database query languages: medium relational calculus and medium relational algebra.

给出了中介关系数据模型，在此基础上构造了两类不同的中介关系数据库查询语言：中介关系演算和中介关系代数。

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The expressive power of XQuery is comparable to the criterion of "relational completeness" defined for database query languages, though no such formal standard has been defined for an XML data model.

XQuery的表达能力可以与为数据库查询语言定义的“关系完整性”标准相比，虽然还没有为XML数据模型定义这样的正式标准。

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