Querying two or more tables at the same data centre

You can make a selective crossmatch of two or more tables if they are all held in the same virtual data centre; that is, if they are in the same list which appears in the Task Runner Query Builder when you select a Queryable database from VOExplorer.

• Example: the LEDAS database contains catalogues from a variety of X-ray and other high energy satellites and a selection of surveys at all wavelengths.

• Use VOExplorer to find the data service just called LEDAS and then construct a query, for example to match radio sources fainter than 5 mJy from FIRST with 2XMM X-ray and EUVE/ROSAT UV detections (note larger search radius for UV sources):

  SELECT TOP 100 * FROM "first" AS a, "2xmmp" AS b, euvexrtcat as c
  WHERE ABS(a.ra-b.ra ) <0.01 AND ABS(a.decl-b.decl) <0.01 
  AND ABS(a.ra-c.ra ) <0.1 AND ABS(a.decl-c.decl) <0.1 
  AND a.flux_20_cm < 5
  

This could be extended to any number of catalogues, time and size permitting. LEDAS currently imposes a limit of 50000 rows returned from any one query.

• Example: the SWIRE Catalogues contain source tables at different wavelengths. This query selects the first 100 sources with positions closer than 0.001 deg in RA and Dec which were detected at 24 and 70 micron.

SELECT TOP 100 * FROM SWIRE2_EN1_24um_23nov05 AS a, 
                 SWIRE2_EN1_70um_23nov05 AS b 
WHERE ABS(a.ra-b.ra ) <0.0010 AND ABS(a."dec"-b."dec" ) <0.0010 

• You can add more conditions such as only selecting objects brighter than a certain limit. See Query Builder help, especially the More advanced queries such as
  • Joining 3 tables using the targetid parameter 6dfthreewayjoin.txt