ADQL

ADQL stands for Astronomical Data Query Language. It is a query language designed for retrieving data from astronomical catalogs and other data services within the Virtual Observatory (VO). ADQL is built as an extension of SQL that preserves the familiar SQL SELECT-FROM-WHERE structure while adding astronomy-specific capabilities.

A key feature is the support for spatial and coordinate-based queries. ADQL introduces region constructors such as CIRCLE, BOX, and POLYGON, and the CONTAINS spatial predicate, allowing users to select objects that lie within a defined region on the sky. Queries commonly reference standard coordinate systems (for example ICRS) and can perform cross-matching and cone searches by combining these constructs with standard SQL predicates.

ADQL is defined and maintained by the International Virtual Observatory Alliance (IVOA) as part of its standards for data access and interoperability. It is implemented by Table Access Protocol (TAP) services and is commonly used to query catalogs, images, and other VO resources. Many VO tools and libraries accept ADQL queries and translate them to backend implementations.

An example ADQL query to find objects near a target position within 0.5 degrees:

SELECT ra, dec, objID

FROM my_catalog

WHERE CONTAINS(POINT('ICRS', ra, dec), CIRCLE('ICRS', 10.684, 41.269, 0.5)) = 1

The language remains flexible, with variants and extensions across services, but the core goal is to enable standardized, interoperable access to astronomical data across the VO ecosystem.