[ADQL] Erratum to ADQL-2.0 for NULL values

[Gregory MANTELET]

Dear DAL members,

Here is a Proposed Erratum aiming to fix ADQL-2.0 about the usage of 
NULL values. Apart from the constraint `IS [NOT] NULL`, it was not 
possible to use the special value NULL anywhere.

This problem has been raised thanks to the development of the ADQL 
validator for the official IVOA grammar: 
https://github.com/ivoa/lyonetia/pull/16#issuecomment-1406523355 One of 
the examples that highlighted this issue is:

SELECT x FROM y WHERE Point(NULL, 2, 3)=x

Allowing NULL instead of a literal, expression or column name is anyway 
already supported in most of the existing ADQL parsers and TAP services 
(DACHS and CADC) and is completely supported by major DBMS.

Here is the link toward this Proposed Erratum:

    https://wiki.ivoa.net/twiki/bin/view/IVOA/ADQL20Err4


Don't hesitate to comment.

Cheers,
Grégory
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