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Ontop

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What Ontop is

Ontop is an open source Virtual Knowledge Graph system that presents relational database content as knowledge graphs while leaving the data in its source databases. It translates SPARQL queries over those graphs into SQL queries executed by the connected relational data sources. Ontop uses RDF 1.1, supports RDFS and OWL 2 QL ontologies, and works with R2RML and Ontop mappings. It supports most SPARQL 1.1 features, including common aggregation and GeoSPARQL functions. Documented database systems include PostgreSQL, MySQL, MariaDB, SQL Server, Oracle, DB2, Snowflake, Databricks, Google BigQuery, AWS Redshift, DuckDB, AWS DynamoDB, and TDengine. Supported federators include Denodo, Dremio, Teiid, Apache Spark, Trino, PrestoDB, and AWS Athena. You can run it as a SPARQL endpoint through its command-line interface or Docker image. Other capabilities include virtual views called lenses, RDF file materialization, and a Protégé plugin for editing and testing mappings. The CLI supports bootstrapping, endpoint startup, metadata extraction, querying, validation, and mapping operations. Ontop is licensed under Apache 2.0 and its distributions are available through GitHub, Docker Hub, SourceForge, and the Protégé plugin repository.

Who it suits

Ontop suits teams and developers who want to query relational data as a virtual knowledge graph with SPARQL while keeping the data in its existing databases. It is also relevant to ontology and mapping work involving its supported RDF formats, database systems, and federators.

What it does well

Keeps graph data in the source databases.
Translates SPARQL queries into SQL for relational sources.
Supports RDFS, OWL 2 QL, R2RML, and Ontop mappings.
Works with a broad list of documented databases and federators.
Apache 2.0 licensed and open source.

What to know first

The Protégé plugin does not support lenses.
Database JDBC drivers must be downloaded and placed in Ontop’s jdbc directory.
Generic JDBC may not handle every database setup or SPARQL query correctly.

Getting started

Get an Ontop distribution from GitHub, Docker Hub, SourceForge, or the Protégé plugin repository.
Download the JDBC driver for your database system and place it in Ontop’s jdbc directory.
Configure mappings and ontology information for the relational data.
Use the command-line interface or Docker image to start a SPARQL endpoint.
Query the virtual graph with SPARQL, or use CLI commands to query, validate mappings, or materialize RDF.

Verdict

Choose Ontop if you need an open source way to expose relational databases as virtual knowledge graphs and query them with SPARQL. Its database driver setup and generic JDBC caveats may be a concern for configurations without a documented dialect-specific connector; users needing lenses in Protégé will also need another editing approach.

Questions

Is Ontop free and open source?

Yes. Ontop is available under the Apache 2.0 license.

What does Ontop do with relational data?

It exposes relational database content as virtual knowledge graphs and translates SPARQL queries into SQL queries run by source databases. The data stays in its source databases.

Which query language does Ontop support?

Ontop supports SPARQL and the majority of SPARQL 1.1 features, including main aggregation functions and GeoSPARQL functions.

Which databases and federators are documented?

Documented databases include PostgreSQL, MySQL, MariaDB, SQL Server, Oracle, DB2, Snowflake, Databricks, Google BigQuery, AWS Redshift, DuckDB, AWS DynamoDB, and TDengine. Supported federators include Denodo, Dremio, Teiid, Apache Spark, Trino, PrestoDB, and AWS Athena.

Can Ontop materialize virtual graphs?

Yes. It can materialize virtual graphs into RDF files.

Does the Protégé plugin support lenses?

No. The plugin supports editing and testing mappings, but not lenses.

Facts from Ontop's own pages

CLI capabilities: The CLI includes commands for bootstrapping, starting an endpoint, extracting database metadata, materializing RDF, querying, validating and manipulating mappings. · ontop-vkg.org, 8 Oct 2026
Database driver requirement: Users must manually download the JDBC driver for their database system and place it in Ontop's jdbc directory. · ontop-vkg.org, 8 Oct 2026
Database integrations: Documented database systems include PostgreSQL, MySQL, MariaDB, SQL Server, Oracle, DB2, Snowflake, Databricks, Google BigQuery, AWS Redshift, DuckDB, AWS DynamoDB, and TDengine. · ontop-vkg.org, 8 Oct 2026
Data handling: The graphs are virtual, so data remains in its source databases rather than being moved to another database. · ontop-vkg.org, 8 Oct 2026
Deployment: Ontop can be deployed as a SPARQL endpoint using its command-line interface or Docker image. · ontop-vkg.org, 8 Oct 2026
Downloads: The project says its distributions are available through GitHub, Docker Hub, SourceForge, and the Protégé plugin repository. · ontop-vkg.org, 8 Oct 2026
Federation integrations: Ontop supports database federators including Denodo, Dremio, Teiid, Apache Spark, Trino, PrestoDB, and AWS Athena. · ontop-vkg.org, 8 Oct 2026
Generic JDBC caveat: Ontop warns that generic JDBC support is not sufficient for any data source and is not guaranteed to handle every database setup and SPARQL query correctly. · ontop-vkg.org, 8 Oct 2026
Generic JDBC limitation: The generic JDBC implementation is not recommended because it may not handle every database setup and SPARQL query correctly; dialect-specific connectors are available for listed systems. · ontop-vkg.org, 8 Oct 2026
Headquarters: Bolzano, Italy · ontop-vkg.org, 28 Sept 2026
JDBC setup: Users need to download the JDBC driver for their database system and place it in Ontop’s jdbc directory. · ontop-vkg.org, 8 Oct 2026
License: Ontop is available under the Apache 2.0 license. · ontop-vkg.org, 8 Oct 2026
Mapping editor: A plugin for the Protégé ontology editor lets users edit and test mappings, but it does not support lenses. · ontop-vkg.org, 8 Oct 2026
Materialization: Ontop can materialize virtual graphs into RDF files. · ontop-vkg.org, 8 Oct 2026
Organizations: The project says it is backed by the Free University of Bozen-Bolzano and Ontopic s.r.l., with regular contributions from Birkbeck, University of London, and Southeast University. · ontop-vkg.org, 8 Oct 2026
Other capabilities: Ontop supports lenses as virtual views, can materialize virtual graphs into RDF files, and provides a Protégé plugin for editing and testing mappings. · ontop-vkg.org, 8 Oct 2026
Plugin limitation: The Protégé plugin does not support lenses; the documentation recommends text editors or commercial offerings for advanced use. · ontop-vkg.org, 8 Oct 2026
Product: Ontop is a Virtual Knowledge Graph system that exposes relational database content as knowledge graphs while leaving data in its source databases. · ontop-vkg.org, 8 Oct 2026
Project and license: Ontop was initiated in 2009 by researchers at the Free University of Bozen-Bolzano and is available under the Apache 2.0 license. · ontop-vkg.org, 8 Oct 2026
Purpose: Ontop exposes relational database contents as virtual knowledge graphs and translates SPARQL queries into SQL queries executed by the source databases. · ontop-vkg.org, 8 Oct 2026
Querying: Ontop translates SPARQL queries over knowledge graphs into SQL queries executed by relational data sources. · ontop-vkg.org, 8 Oct 2026
SPARQL support: It supports the majority of SPARQL 1.1 features, including main aggregation functions and GeoSPARQL functions. · ontop-vkg.org, 8 Oct 2026
Standards: It uses RDF 1.1, supports RDFS and OWL 2 QL ontologies, and supports R2RML and Ontop mappings. · ontop-vkg.org, 8 Oct 2026
Support and commercial offerings: The documentation says Ontopic provides a suite of solutions for leveraging Ontop and Virtual Knowledge Graphs; it does not state a support price or service level on that page. · ontop-vkg.org, 8 Oct 2026