Commonly Searched Columns
The Schema Browser contains complete documentation on
every table in the SkyServer database. But, most of the queries you
will write will search through only a small subset of those tables.
And even the commonly used tables contain many technical, esoteric
items - most users will search for only a few columns.
Below is a list of some of the most commonly used tables (or views) and a short
description of them. Click on any of the table names to be taken to a list
of the most commonly used columns in that table. Take some time to
understand what these columns mean, because you will use them over
and over again in your queries. Click on a table (or view) name in the list
below to launch a popup window showing the most commonly-searched data
columns in that table/view.
PhotoObj - stores information about the images of
every object, including run, rerun, camcol, field, ra, dec, magnitudes and
object flags.
PlateX - stores information on the aluminum plates that the SDSS
uses to take spectra, including their exposure times and reddening
information. You will need to find the Plate and MJD in this table to
look up an object's spectrum in the Get Spectra tool.
SpecObj - stores information on objects' spectra,
including redshifts and spectroscopic classifications.
In addition, SkyServer contains several subsets of the PhotoObj table.
PhotoPrimary contains only the "best"
measurements of each object. Generally, it's better to use
PhotoPrimary rather than PhotoObj, which contains both good and bad
data. Star contains
only data for stars, Galaxy contains only
data for galaxies, and Unknown contains
only data for objects classified as "unknown." These subsets are actually views
rather than tables; you will learn the difference later in the
tutorial.
Two Other Tips
First, SkyServer's Query tool is limited to 90
seconds of search time and 100,000 results. If you want to write
queries that return more than 100,000 objects, you have to be clever:
split your query into multiple pieces. For example, if you are looking
for stars between ra = 0 and ra = 3, you could write two queries,
one from ra = 0 to ra = 1.5 and one from ra = 1.5 to ra = 3. Save
all the results of the queries until you have all the results you
want.
Or, you can get around the limit
completely writing your query in
CasJobs  ,
which has no limits on search time or number of results.
If you want the query to return fewer results, you can add
the statement "top n" to the select block, where n is some
integer from 1 to 1,000.
For example, when you're testing to see if a query returns reasonable
results, you might not want to have to wade through 1,000 records. You
might want to have the query return 10 records instead; use "select top 10".
Second, SQL is not case-sensitive, meaning capital and lowercase letters
are the same. The commands "select," "Select," "SELECT" will all do the same thing.
But, it's a good idea to use capitalization and spacing to make your queries as easy
as possible for other people to understand.
Click Next to learn more about how to write SQL queries.
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