ADQL (Astronomical Data Query Language; http://www.ivoa.net/documents/latest/ADQL.html) is based on SQL92 and defines a set of restrictions and extensions to support generic and astronomy specific operations. It is used extensively with TAP (Table Access Protocol; http://www.ivoa.net/documents/TAP/20100327). ADQL is implementation agnostic so SQL extensions specific to a particular RDBMS, such as the q3c spatial indexing for Postgres, are not valid within ADQL queries. Perhaps of most use are the geometric functions which ADQL defines and can be included in a query statement; for example,
select * from some.table where CONTAINS(POINT('J2000',ra,dec),CIRCLE('J2000',121.17,-21.57,1.0))=1
will find all objects which lie within a circle of radius 1 of the specified point (see http://tapvizier.u-strasbg.fr/adql/help.html for further examples).