|Introduction to SQL|
|Form Query User Guide|
|SkyServer Traffic Page|
|Contact Help Desk|
|2. A Simple Query|
|3. Common Searches|
|4. More Samples|
|5. Multiple Tables|
|6. Aggregate Fcns.|
|7. Group By|
|8. Order By|
All the queries you have written so far return every record that matches the criteria in the where block. But with SQL, you can also return statistical summaries of all matching records. For example, look at the query below:
The query retrieves the minimum, maximum, and average declination of one of the SDSS's equatorial stripes (the area of the survey near the celestial equator, dec = 0). The commands min(x), max(x), and avg(x) are aggregate functions, named because they operate on an aggregate, or sum, of all the matching records. SQL's aggregate functions are listed in the table below:
The difference between count(x) and count(*) can appear confusing, but they are usually used in different situations. Use count(*) to find out the number of records in a table - how big the table is. The command "select count(*) from specObj" returns 163,901 - meaning there are 163,901 separate records in the specObj table. The command "select count(*) from PhotoPrimary" returns 52,530,681.
Unlike count(*), count(x) is usually used with a characteristic listed in the where block. For example, the query below returns the number of records in the specObj table that have redshift between 0.5 and 1: 4,577. So the number of objects with redshift between 0.5 and 1 found by count(x) is much less than the total number of objects in specObj, found by count(*).
Try pressing the buttons "Query 1" and "Query 2" below. These buttons will make the two queries above (in the purple boxes) appear in the query window. Press Submit to execute the queries. Do you get what you expected? Do the results seem reasonable to you?
Click Next when you are ready to move on.