Language-integrated query based on comprehension syntax is a powerful technique for safe database programming, and provides a basis for advanced techniques such as query shredding or query flattening that allow efficient programming with complex nested collections. However, the foundations of these techniques are lacking: although SQL, the most widely-used database query language, supports heterogeneous queries that mix set and multiset semantics, these important capabilities are not supported by known correctness results or implementations that assume homogeneous collections. In this paper we study language-integrated query for a heterogeneous query language $NRC_\lambda(Set,Bag)$ that combines set and multiset constructs. We show how to normalize and translate queries to SQL, and develop a novel approach to querying heterogeneous nested collections, based on the insight that ``local'' query subexpressions that calculate nested subcollections can be ``lifted'' to the top level analogously to lambda-lifting for local function definitions.

中文翻译：

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查询提升：针对异构嵌套集合的语言集成查询

基于理解语法的语言集成查询是用于安全数据库编程的强大技术，并且为高级技术（例如查询分解或查询展平）提供了基础，这些高级技术可以对复杂的嵌套集合进行高效编程。但是，缺乏这些技术的基础：尽管SQL是使用最广泛的数据库查询语言，但它支持混合了集合和多集语义的异构查询，但是已知的正确性结果或假定同类集合的实现均不支持这些重要功能。在本文中，我们研究了将集合和多集构造相结合的异构查询语言$ NRC_ \ lambda（Set，Bag）$的语言集成查询。我们展示了如何规范化查询并将查询转换为SQL，