Query Interfaces - Clive Page - VOQL Working Group - 2003 May 13.

   What is a query language for?

For the VO portal to transfer a query to a remote data service?
For power user to enter general query specification, say in a text-box?

   Simple Queries

Simple image access, querying a particular image archive
similar to cone-search except specifies box rather than circle

Search for observational data sets (raw or semi-processed data)
Cone search on specified observation log
Results not a table but (often) a compressed TAR file

Cone-search of a particular source catalogue
Specify position (RA,DEC), radius, maybe other selection constraints using
column names

   Resource Discovery Query

For example: 
locate datasets with data on objects 
observed in <waveband> 
in <area of sky> 
with sensitivity at least <magnitude limit>.  

Hard to handle at present, will be sent to VO Registry
Do we need a separate registry query language?

   Extend Cone-search to multiple catalogues

Can be done by Vizier 
All tables co-located so no network overhead
Additional constraints use column names, so laborious to specify for
multiple tables
Also done by Astrobrowse (but only simple cone-search)
Problems:
How to specify additional constraints?
Column names unknown, can we use UCDs but they are not uniquely mapped to
columns?
How to handle merger of results from heterogeneous tables?


   Cross-matching query

urrently difficult and therefore rare:
Astrobrowse (for a few specific table pairs)
JHU Skyquery can match SDSS/2MASS/FIRST (but only for small patches of
sky).
Query must specify:
List of tables to match in some search order
Matching criterion (positional errors + some epsilon?)
Columns required in the resulting table
+ distance between matched sources?

   Search Parameters  simple cone search

Object name or celestial position
Name resolver or Equinox
Shape of search area (circle/box/polygon/ellipse)
Size of search area (radius/dimensions)
Limits on named columns (or UCDs)
VMAG between 18.5 and 19.5
Waveband of interest (radio, gamma-ray, etc)
List of sites to search 
General SQL WHERE-clause

   Output Options

Data format
Images (FITS/GIF/JPEG)
Tables (FITS, HTML, VOTable, CSV, plain text)
Which column names (or UCDs) to return
Maximum number of rows to return
Coordinates to use (Celestial/Galactic) and equinox
Ordering: sort ascending/descending by some column
Show UCDs and units as well as names

   Query language considerations

Large choice of XML-based query languages
Xpath, Xquery, XqueryX, Xupdate, Quilt, XML-QL, XQL, and probably others
SQL used by many (not all) current archives
Xquery draft W3C standard: 
has WHERE-clause similar to SQL

Needs special function for cone-search 
For example  CONE(ra, dec, radius)