Querying from multiple tables: Joining

Take another look at the Schema Browser. Data on the same objects are often stored in different tables. For example, a galaxy's magnitudes are stored in the photoObj table, its redshift in the specObj table, and the plate used to observe its spectrum in the plateX table.

Quite often, you will want to compare information from many different tables. For example, making a Hubble Diagram to show the expansion of the universe requires some measure of brightness, such as magnitude, and redshift. Magnitude is in the photoObj table; redshift is in the specObj table. How do you search both tables?

You can list more than one table in the from block, for example "from photoObj, specObj." But be careful - you have to be sure you are looking at the same object in both tables! To make sure you are searching the same objects, you must use a join.

For example, look at the following query:

To join two tables, look for common entries - when the same column appears in both tables. If you match the same column in the two different tables, then only records with the same data value in that column will be returned. Since you are ensuring that records contain the same data for this column, can be sure that you are looking at data for the same object in both tables. In the example above, what statement joins the photoObj and specObj tables?   Answer

To find columns to use in joining tables, look in the Schema Browser. Find places where the same column name appears in more than one table. You can use that column in the where block to join the tables.

Notice that when you are querying multiple tables at once, you must specify the table from which each piece of data is retrieved using the format "table.column". For example, plateX.plateID retrieves the value in the column plateID from the table plate. You must declare all tables you are searching in the from block, separating them with commas.

Naming tables in the select and where blocks is important because sometimes columns in different tables have the same name, yet contain different data. For example, the PhotoObj table contains the right ascension and declination of the object in the "ra" and "dec" columns. The PlateX table also has ra and dec columns, but these are the ra and dec of the telescope while it is observing a single plate, not the ra and dec of an individual object on the plate. So photoObj.ra and plate.ra mean totally different things!

To save typing, you can also declare "nicknames" for tables in the from block. By saying "from photoObj p, specObj s", you can use the nicknames p and s in the rest of your query - typing p.ra or s.zConf. Whether you want to use nicknames or full names for tables is up to you.

In addition to the join statements, the query above includes two other commands in the where block. The command specObj.specClass = 3 tells the query to return quasars only, and the command specObj.zconf > .95 tells the query to return only objects for which we have well-measured redshifts. Again, you can add more restrictions to your query in terms of colors, magnitudes, ra and dec or any number of parameters.

Try It!

Click Submit below to run the query. If you would like, try rewriting the query using one-letter nicknames for all the tables.

Click Next to get some more practice using joins.