COALESECE in ADQL?

[Markus Demleitner]

Dear DAL folks,

In the context of my work on improving support for data discovery
(rather than service discovery) in pyVO (cf.
<https://github.com/msdemlei/pyvo/tree/add-discoverdata>), I've found
that writing queries that do that without SQL COALESCE is really hard
given the way RegTAP is written (using NULLs for missing access_urls,
standardIds, and such).

Since I consider data discovery (as per DDC
<https://ivoa.net/documents/discovercollections>) a mode of registry
use the role of which will grow significantly, I am fairly determined
to require COALESCE for RegTAP 1.2.  That requirement is a lot easier
to state if ADQL itself already mentions COALESCE.

So, although I certainly do not want to further delay ADQL RFC, I've
put in a relatively compact PR,
https://github.com/ivoa-std/ADQL/pull/69 -- perhaps it could still
sneak in?

Thanks,

         Markus