|Introduction to SQL
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SkyServer is the education and outreach web site of the Sloan Digital Sky Survey (SDSS). SkyServer makes the entire survey available, free of charge, to the public. With SkyServer, you can study exactly the same stars and galaxies that professional astronomers are studying right now.
The SDSS is one of the most ambitious scientific projects of all time. Its goal is to make a high-quality three-dimensional map of the universe. The survey uses a specially-built 2.5-meter telescope in New Mexico, a CCD camera, and sophisticated software to store and analyze its data.
The SDSS began in June 1998. By the time it ends, the SDSS will have mapped 25% of the night sky, taking images of over 100 million objects. The survey has already completed a preliminary map of the universe: you can see the map in our First Discoveries section.
The main menu bar near the top of the screen (from Home to Help) is always available to navigate the site. In addition, when you browse using the menubar, a side menu comes up to help you navigate different parts of the site.
SkyServer offers two types of data: images and spectra. Images are pictures of the night sky taken by our digital camera. Spectra are measurements of the amount of light a star or galaxy gives off at different wavelengths. SkyServer has images of more than 80 million stars and galaxies, and spectra for more than 180,000. For more on our data, see the Getting Started pages.
The SDSS takes data in long, narrow "stripes." The current data includes several stripes near the celestial equator (the Earth's equator projected into the sky), and several in a C-shaped band of the northern sky. See the DR1 Sky Coverage page for maps and tables of our sky coverage. To see if a specific area is part of the SDSS, enter its coordinates into the Finding Chart.
Over the next few years, we will add more data to SkyServer until we have about one-quarter of the sky covered.
Here is a quick summary of what the most commonly-used tools do. Famous Places is a gallery of beautiful SDSS images. The Navigation tool lets you point and click through the sky. The Object Explorer gives you access to complete data on a single star, galaxy, or quasar. The Search tools let you see data on all objects in a certain part of sky. Or, if you know SQL, you can use the search tools to return all objects that meet whatever criteria you can think of.
SkyServer has an extensive Help section, including a Glossary and How-To Tutorials. The Schema Browser is essential for using the Search tool. SkyServer's About Astronomy and About SDSS sections also contain readings that help explain concepts from astronomy and the SDSS.
SkyServer's Projects use SDSS data to interactively teach astronomy. With our Projects, you can learn about the evolution of stars, the types of galaxies, the history of the universe, and much more.
Teachers, we welcome you to use and adapt any of our projects in your classes, free of charge. For more information on what you can do with SkyServer in the classroom, see our Teacher FAQ.
The SpecObjAll table contains ALL spectroscopic objects, regardless of their status in the survey. Queries on this table can produces unusual or undesired objects. Thus, we have created the SpecObj view, which contains data for ONLY those fibers defined as SciencePrimary. To be SciencePrimary, an object must meet all of the following criteria:
As a result, some plates may have many (or even all) of their fibers excluded from SpecObj. Some of these instances are:
Note: The definition of SciencePrimary relies purely on spectroscopic and geometric considerations. There are objects in SpecObj which do not have a corresponding Best photometric object (406 in DR1).
PhotoObjAll is a table in the Best and Target databases which contains
all of the measured photometric quantities for all of the imaging objects.
Because we measure so many different parameters for each object, this is a
very large table, and queries can take very long to run.
In addition, we have created a view ofPhotoObjAll which contains only those objects which are Primary or Secondary. This view is called PhotoObj. Because this view effectively contains fewer objects than PhotoObjAll, but all the measured quantities for these objects, queries will execute faster.
Given the above, a user should:
Because PhotoTag has many fewer parameters, larger portions of it can be cached, improving performance. We have found that for almost all queries which contain parameters fully in PhotoTag, it is faster. In the case of looking for objects that are detected multiple times, one could perform a join on PhotoTag with itself, requiring that one object be Primary and the other Secondary.
The SpecPhotoAll table is a precomputed join between the Best PhotoObjAll and SpecObjAll tables. It includes the most requested parameters from these two tables, as well as a few pieces of information about tiling. It also includes the TargetObjID, which allows user to retrieve the Target version of the photometry.
As described above, The SpecPhotoAll is a precomputed join between the Best PhotoObjAll and SpecObjAll tables. This includes all non-science objects, and a variety of objects many users will not be interested in. The SpecPhoto view includes only those pairs where the SpecObj is a sciencePrimary (see the definition above), and the BEST PhotoObj is a PRIMARY.
Internally, these numbers are stored to their full precision as they come out of the spectroscopic pipeline. When you perform a query, they have some default string format applied that cuts them to what you see. But you can use the str() function to change the string format to whatever you like.
To get z to 6 decimals, e.g., change your query to 'select str(z,8,6) as z' instead of just z, and analogously for zErr. This applies the function str() to the values in column z and returns the result with column label z (without the "as", the result of a function has no column label). The str(col,length,dec) function takes the numerical value in 'col' and formats it as a string of length 'length' and with 'dec' significant digits, i.e. it's the SQL equivalent to the C function printf("%8.6f",z). str() rounds the result to the number of decimals you request.
See our Credits page.
Do you have other questions that aren't answered here? E-mail them to us!