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Encyclopedia of Database Systems pp 74–79Cite as

Aggregation: Expressiveness and Containment

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Definition

An aggregate function is a function that receives as input a multiset of values and returns a single value. For example, the aggregate function count returns the number of input values. An aggregate query is simply a query that mentions an aggregate function, usually as part of its output. Aggregate queries are commonly used to retrieve concise information from a database, since they can cover many data items while returning few. Aggregation is allowed in SQL, and the addition of aggregation to other query languages, such as relational algebra and Datalog, has been studied.

The problem of determining query expressiveness is to characterize the types of queries that can be expressed in a given query language. The study of query expressiveness for languages with aggregation is often focused on determining how aggregation increases the ability to formulate queries. It has been shown that relational algebra with aggregation (which models SQL) has a locality property.

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Recommended Reading

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Author information

Authors and Affiliations

  1. The Rachel and Selim Benin School of Computer Science and Engineering, The Hebrew University of Jerusalem, Jerusalem, Israel

    Sara Cohen

Authors
  1. Sara Cohen
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Correspondence to Sara Cohen .

Editor information

Editors and Affiliations

  1. Georgia Institute of Technology College of Computing, Atlanta, GA, USA

    Ling Liu

  2. University of Waterloo School of Computer Science, Waterloo, ON, Canada

    M. Tamer Özsu

Section Editor information

  1. School of Informatics, University of Edinburgh, Edinburgh, Scotland, UK

    Leonid Libkin

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Cohen, S. (2018). Aggregation: Expressiveness and Containment. In: Liu, L., Özsu, M.T. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8265-9_1256

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