Graph Query Language: Difference between revisions

Revision as of 00:29, 11 November 2019

Revision as of 01:02, 11 November 2019

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==Extending existing graph query languages==

The GQL project draws on multiple sources or inputs, notably existing industrial languages and a new section of the SQL standard. In preparatory discussions within WG3 surveys of the history<ref name="GQLs history">{{cite web|url=https://s3.amazonaws.com/artifacts.opencypher.org/website/materials/DM32.2/DM32.2-2018-00085R1-recent_history_of_property_graph_query_languages.pdf|title=''An overview of the recent history of Graph Query Languages''|last=Lindaaker|first=Tobias|date=May 2018|website=|publisher=opencypher.org|accessdate=October 6, 2019}}</ref> and comparative content of some of these inputs<ref name="Summary Chart">{{cite web|url=https://s3.amazonaws.com/artifacts.opencypher.org/website/materials/DM32.2/DM32.2-2018-00086r1-summary-chart-of-cypher-pgql-gcore.pdf|title=''Summary Chart of Cypher, PGQL, and G-Core''|last=Plantikow|first=Stefan|date=May 2018|website=|publisher=opencypher.org|accessdate=November 3, 2019}}</ref> were presented. GQL will be a declarative language with its own distinct syntax, playing a similar role to SQL in the building of a database application. Other graph query languages have been defined which offer direct procedural features such as branching and looping[ (Apache Tinkerpop's Gremlin<ref name="Gremlin 2015">{{cite web|url=https://dl.acm.org/citation.cfm?id=2815073|title=''REFThe TINKERPOPGremlin graph traversal machine and language (invited talk). In Proceedings of the 15th Symposium on Database Programming Languages (DBPL 2015). ACM, GREMLIN''']New York, ['''REFNY, USA, 1-10. DOI: GQSL10.1145/2815072.2815073''']|last=Rodriguez|first=Marko A.|date=2015|website=|publisher=ACM|accessdate=November 10, 2019}}</ref>, GSQL<ref name="GSQL white paper"/>, and the ability to traverse a graph iteratively['''REF TINKERPOP, GREMLIN'''], but GQL will not incorporate such features. <ref name="Wood Graph Query">{{cite web|url=https://dl.acm.org/ft_gateway.cfm?id=2206879&ftid=1212198&dwn=1&CFID=106456621&CFTOKEN=92daacebb1744396-220C1ECA-F98C-4552-03541423FDE04D18|title=''Query languages for graph databases. '', SIGMOD Rec. 41, 1 (April 2012), 50-60. DOI: 10.1145/2206869.2206879.|last=Wood|first=Peter T.|date=|website=|publisher=ACM|accessdate=October 25, 2019}}</ref>['''REF MARCELO''']. However, GQL is envisaged as a specific case of a more general class of graph languages, which will share a graph type system and a calling interface for procedures that process graphs.

=== SQL/PGQ Property Graph Query ===

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Cypher allows creation, reading, updating and deleting of graph elements, and is a language that can therefore be used for analytics engines and transactional databases.

Cypher uses compact fixed- and variable-length patterns which combine node and relationship (edge) topology representations with label existence and property value predicates. By matching such a pattern against graph data elements, a query can extract references to nodes, relationships and paths of interest. Those references are emitted as a "binding table" where column names are bound to a setmultiset of graph elements. The name of a column becomes the name of a "binding variable", whose value is a specific graph element reference for each row of the table.

For example, a pattern &nbsp;{{code|MATCH (p:Person}-[LIVES_IN]->(c:City)}}&nbsp; will generate a two-column output table. The first column named &nbsp;{{code|p}}&nbsp; will contain references to nodes with a label &nbsp;{{code|Person}}&nbsp;. For each row theThe second column named &nbsp;{{code|c}}&nbsp; will contain references to nodes with a label &nbsp;{{code|City}}&nbsp;, denoting the city where the person lives.

The binding variables &nbsp;{{code|p}}&nbsp; and &nbsp;{{code|c}}&nbsp; can then be dereferenced to obtain access to property values associated with the elementelements referred to by a variable. The example query might be terminated with a &nbsp;{{code|RETURN}}, resulting in a complete query like this:

<pre>MATCH (p:Person}-[LIVES_IN]->(c:City)