1. A few notes on JVOQL François Ochsenbein

2. JVOQL: a few questions FROM clause: not clear how to specify the list e.g. FROM clause: not clear how to specify the list e.g. catalogues from some author ? catalogues from some author ? catalogues covering some wavelength range ? catalogues covering some wavelength range ? must the conditions be known by the registry ? must the conditions be known by the registry ? Non-SQL examples Non-SQL examples create view as select * from table1 where [not] exist table2 create view as select * from table1 union select * from table2

3. UCD Usage in JVOQL UCDs enable the possibility of generic queries (column names are not known!) UCDs enable the possibility of generic queries (column names are not known!) Give all objects having a V magnitude in a range Give all objects having a V magnitude in a range Give all known quasars with redshift > 1 Give all known quasars with redshift > 1 Used in Aladin for 'filtering' (graphical representation) Used in Aladin for 'filtering' (graphical representation) circles with radius proportionnal to brightness circles with radius proportionnal to brightness draw ellipses from major/minor axises + PA draw ellipses from major/minor axises + PA use a color function of V-R use a color function of V-R

5. But UCDs are not unique ! Several columns of a catalog may share the same UCD measurements at different epochs measurements at different epochs measurements coming from different origins (e.g. USNO-B1) measurements coming from different origins (e.g. USNO-B1) The default rule could be: -- Take the first column matching the UCD -- or better, take the first column with not-null contents

6. Questions about XMATCH XMATCH(a,b,...) < something Can the something be in the table (e.g. 3sigmas) or some combination of the error ellipses ? Can the something be in the table (e.g. 3sigmas) or some combination of the error ellipses ? limited to positional criteria ? limited to positional criteria ?