Query languages are computer languages used to make queries into databases and information systems.

Broadly, query languages can be classified according to whether they are database query languages or information retrieval query languages. Examples include:

• .QL is a proprietary object-oriented query language for querying relational databases;
• Common Query Language (CQL) a formal language for representing queries to information retrieval systems such as as web indexes or bibliographic catalogues.
• CODASYL;
• D is a query language for truly relational database management systems (TRDBMS);
• DMX is a query language for Data Mining models;
• Datalog is a query language for deductive databases;
• ERROL is a query language over the Entity-relationship model (ERM) which mimics major Natural language constructs (of the English language and possibly other languages). It is especially tailored for relational databases;
• Gellish English is a language that can be used for queries in Gellish English Databases [1], for dialogues (requests and responses) as well as for information modeling and knowledge modeling;
• ISBL is a query language for PRTV, one of the earliest relational database management systems;
• LDAP is an application protocol for querying and modifying directory services running over TCP/IP.
• MQL is a cheminformatics query language for a substructure search allowing beside nominal properties also numerical properties;
• MDX is a query language for OLAP databases;
• OQL is Object Query Language;
• OCL (Object Constraint Language). Despite it's name, OCL is also an object query language and a OMG standard.
• OPath, intended for use in querying WinFS Stores;
• Poliqarp Query Language is a special query language designed to analyze annotated text. Used in the Poliqarp search engine;
• QUEL is a relational database access language, similar in most ways to SQL;
• SMARTS is the cheminformatics standard for a substructure search;
• SPARQL is a query language for RDF graphs;
• SQL is a well known query language for relational databases;
• SuprTool is a proprietary query language for SuprTool, a database access program used for accessing data in Image/SQL (TurboIMAGE) and Oracle databases;
• TMQL Topic Map Query Language is a query language for Topic Maps;
• XQuery is a query language for XML data sources;
• XPath is a language for navigating XML documents;

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