ADQL is a language defined by the IVOA for querying astronomical data. It stands for Astronomical Data Query Language. It is a fork of SQL-92 in which only the query features are used. Astronomical functions and operators have been added, in particular to query data by position. This language is mainly used in the IVOA protocol called TAP for querying relational astronomical data.

Until now, the ADQL grammar has been described in a BNF (Backus-Naur-Form)-inspired formalism, largely following SQL-92 itself. However, in its current form it is not actually used by any implementation for several reasons, including some minor mistakes and lacunae, which were in practice filled borrowing from other SQL implementations. Also, the lack of stringent tokenisation rules resulted in differing interpretations of corner cases (e.g. the string literal '49a').

In the next version of ADQL, it is therefore proposed to change the ADQL language notation from BNF to PEG (Parsing Expression Grammar). In PEG, parsing and tokenisation are specified in a uniform way, and PEG's own grammar is standardised sufficiently well that multiple interoperating implementations can be used off the shelf to obtain parse trees of ADQL clauses (at least conceptually).

This poster aims to show what are the difference between the two notations and what improvements this implies for ADQL.