Databases and SQL

Filtering

One of the most powerful features of a database is the ability to filter data, i.e., to select only those records that match certain criteria. For example, suppose we want to see when a particular site was visited. We can select these records from the Visited table by using a WHERE clause in our query:

SELECT * FROM Visited WHERE site='DR-1';

The database manager executes this query in two stages. First, it checks at each row in the Visited table to see which ones satisfy the WHERE. It then uses the column names following the SELECT keyword to determine what columns to display.

This processing order means that we can filter records using WHERE based on values in columns that aren't then displayed:

SELECT id FROM Visited WHERE site='DR-1';

We can use many other Boolean operators to filter our data. For example, we can ask for all information from the DR-1 site collected before 1930:

SELECT * FROM Visited WHERE site='DR-1' AND dated<'1930-01-01';

If we want to find out what measurements were taken by either Lake or Roerich, we can combine the tests on their names using OR:

SELECT * FROM Survey WHERE person='lake' OR person='roe';

Alternatively, we can use IN to see if a value is in a specific set:

SELECT * FROM Survey WHERE person IN ('lake', 'roe');

We can combine AND with OR, but we need to be careful about which operator is executed first. If we don't use parentheses, we get this:

SELECT * FROM Survey WHERE quant='sal' AND person='lake' OR person='roe';

which is salinity measurements by Lake, and any measurement by Roerich. We probably want this instead:

SELECT * FROM Survey WHERE quant='sal' AND (person='lake' OR person='roe');

We can also filter by partial matches. For example, if we want to know something just about the site names beginning with "DR" we can use the LIKE keyword. The percent symbol acts as a wildcard, matching any characters in that place. It can be used at the beginning, middle, or end of the string:

SELECT * FROM Visited WHERE site LIKE 'DR%';

There are many, many ways you can manipulate queries in SQL (selecting only DISTINCT values; aggregating with GROUP BY; combining queries with UNION; scaling and rounding numerical values; concatenating, searching, and slicing strings; etc.). Instead of covering them all now, we'll hit the most common ones and leave you to learn the rest when you find yourself needing them. Search engines are great for this (for example "sql select unique values" will give you lots of hits about DISTINCT), and the documentation for your database manager are also a good resource. For example, the LIKE docs referenced earlier does a good job laying out the syntax and effect of the LIKE operator (I found the relevant docs by asking a search engine for "sqlite like comparison"). When the offical documentation is too dense, you can almost always find a good blog post or tutorial that covers a particular construct at a slower pace and a higher level. That will give a general framework for the idea, and when you go back to the docs they should make more sense.

SELECT * FROM Site WHERE (lat > -60) OR (lat < 60);