Queries

SkyQuery uses a modified flavour of the SQL language based on Microsoft SQL Server's syntax, just like SkyServer. The current version supports single SELECT statements only.

The following simple query can be used to test basic functionality. It should return a few objects from the SDSS DR7 photometric catalog.

Queries are written using the query editor. Use the 'syntax check' button to verify typos before submitting a query. Queries are always queued for execution to avoid the flooding of the system by too many users. There are two queues: a quick queue for almost instantanious execution of short queries and a long queue for long-running complex queries. To send a query into the quick queue, simply click on the 'quick' button. Result will appear on the bottom of the screen but also get stored in the table MyDB:quickresults. Because quick queries are also subject to queueing, a couple of seconds might elapse before execution begins. Long queries can be executed by clicking the 'execute' button. Results from long queries are always stored in MyDB. To explicitely direct query results to a given table, enter the name of the target table in the form, or use the INTO synstax as follows.

To filter output, use the WHERE clause.

Cross-match queries use extensions to the SQL language to formulate the cross-match problem. Tables have to be listed after the FROM keyword and combined using the CROSS JOIN operator. An XMATCH clause at the end of the query is then used to set the cross-matching criteria. The following query cross-matches the SDSS DR7 catalog with Galex in a small area of the sky. Multi-catalog and full-catalog cross-matches are also possible. Most catalogs denote invalid values by -9999, so filtering out wrong coordinates is always necessary. Also, restricting all catalogs to a small area results in significantly faster query execution.

This query cross-matches objects from the SDSS DR7 and the Galex catalog based on the equatorial J2000 coordinates ra, dec, assuming an astrometric error of 0.1 and 0.2 for SDSS and Galex, respectively. Cross-matching is limited to the 0 < ra < 0.5 degree interval. Conditions set in the form of coord > -9999 are to filter out wrong measurements.

Cross-matching uses a bayesian algorithm to calculate possible matches. Because priors may vary from problem to problem, cuts are made on the Bayes factor intead of the posterior probability. The limit on the Bayes factor is defined in the LIMIT BAYESFACTOR TO XXX expression. A value of 10³ or higher usually means a good match.

In the latter query, because multiple catalog tables are referenced and each table has columns with the same name (ra, dec, etc.), aliases for the tables are defined by appending 'AS alias' after the table name when listed in the FROM clause. Average coordinates are also calculated and can be accessed as a computed table. The 'x' after the XMATCH ( ... ) part defines the alias for this computed table. The computed table has the following columns: ra, dec, cx, cy, cz, logBF, which are right ascension, declination, cartesian unit vector coordinates, and logarithm of the Bayes factor, respectively.