Editing Declarative programming (section)

===Datalog===
The [[Datalog#History |origins of Datalog]] date back to the beginning of logic programming, but it was identified as a separate area around 1977. [[Syntax and semantics of logic programming |Syntactically and semantically]], it is a subset of Prolog. But because it does not have [[Prolog#Data types |compound terms]], it is not [[Turing completeness|Turing-complete]].

Most Datalog systems execute programs bottom-up, using rules to [[forward chaining |reason forwards]], deriving new facts from existing facts, and terminating when there are no new facts that can be derived, or when the derived facts unify with the query. In the above example, a typical Datalog system would first derive the new facts:
<syntaxhighlight lang="prolog">
animal(tom).
animal(jerry).
big(tom).
small(jerry).
</syntaxhighlight>

Using these facts, it would then derive the additional fact:
<syntaxhighlight lang="prolog">
eats(tom, jerry).
</syntaxhighlight>

It would then terminate, both because no new, additional facts can be derived, and because the newly derived fact unifies with the query <syntaxhighlight lang="prolog">
eats(X, jerry).</syntaxhighlight>

Datalog has been applied to such problems as [[data integration]], [[information extraction]], [[Computer network|networking]], [[security]], [[cloud computing]] and [[machine learning]].<ref>{{cite conference | url = http://www.cs.ucdavis.edu/~green/papers/sigmod906t-huang.pdf | conference = SIGMOD 2011 | title = Datalog and Emerging applications | last1 = Huang | first1 = Shan Shan | last2 = Green | first2 = Todd J. | last3 = Loo | first3 = Boon Thau | publisher = Association for Computing Machinery | date = June 12–16, 2011 | location = Athens, Greece | isbn = 978-1-4503-0661-4 | access-date = 2023-08-13 | archive-date = 2020-10-22 | archive-url = https://web.archive.org/web/20201022234145/https://www.cs.ucdavis.edu/~green/papers/sigmod906t-huang.pdf | url-status = live }}</ref><ref>{{Cite conference|title=Neural Datalog Through Time: Informed Temporal Modeling via Logical Specification|book-title=Proceedings of ICML 2020|last1=Mei |first1=Hongyuan |last2=Qin |first2=Guanghui |last3=Xu |first3=Minjie |last4=Eisner |first4=Jason |year=2020 |arxiv=2006.16723 }}</ref>