Expressive Power of Query Languages
The study of expressive power concentrates on comparing classes of queries that can be expressed in different languages, and on proving expressibility - or inexpressibility - of certain queries in a query language.
What queries cannot be expressed in a language ℒ
Which methods are available for proving such results?
Furthermore, if there are two query languages ℒ1 and ℒ2, one may want to compare their expressiveness: for example, ℒ1 ⊈ ℒ2 means that all queries expressible in ℒ1 are also expressible in ℒ2, but there are queries expressible in ℒ2 that are not expressible in ℒ1.
In 1975, Fagin  showed that queries such as the transitive closure of a graph and connectivity...
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