OverviewTeaching: 10 min
Exercises: 10 minQuestions
How can I select subsets of data?Objectives
Write queries that select records that satisfy user-specified conditions.
Explain the order in which the clauses in a query are executed.
One of the most powerful features of a database is
the ability to filter data,
to select only those records that match certain criteria.
suppose we want to see when a particular site was visited.
We can select these records from the
by using a
WHERE clause in our query:
SELECT * FROM Visited WHERE site='DR-1';
The database manager executes this query in two stages.
it checks at each row in the
to see which ones satisfy the
It then uses the column names following the
to determine which columns to display.
This processing order means that
we can filter records using
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';
Most database managers have a special data type for dates. In fact, many have two: one for dates, such as “May 31, 1971”, and one for durations, such as “31 days”. SQLite doesn’t: instead, it stores dates as either text (in the ISO-8601 standard format “YYYY-MM-DD HH:MM:SS.SSSS”), real numbers (the number of days since November 24, 4714 BCE), or integers (the number of seconds since midnight, January 1, 1970). If this sounds complicated, it is, but not nearly as complicated as figuring out historical dates in Sweden.
If we want to find out what measurements were taken by either Lake or Roerich,
we can combine the tests on their names using
SELECT * FROM Survey WHERE person='lake' OR person='roe';
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
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
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%';
we can use
to give a second level of filtering:
SELECT DISTINCT person, quant FROM Survey WHERE person='lake' OR person='roe';
DISTINCT is applied to the values displayed in the chosen columns,
not to the entire rows as they are being processed.
What we have just done is how most people “grow” their SQL queries. We started with something simple that did part of what we wanted, then added more clauses one by one, testing their effects as we went. This is a good strategy — in fact, for complex queries it’s often the only strategy — but it depends on quick turnaround, and on us recognizing the right answer when we get it.
The best way to achieve quick turnaround is often to put a subset of data in a temporary database and run our queries against that, or to fill a small database with synthesized records. For example, instead of trying our queries against an actual database of 20 million Australians, we could run it against a sample of ten thousand, or write a small program to generate ten thousand random (but plausible) records and use that.
Fix This Query
Suppose we want to select all sites that lie more than 42 degrees from the poles. Our first query is:
SELECT * FROM Site WHERE (lat > -48) OR (lat < 48);
Explain why this is wrong, and rewrite the query so that it is correct.
Normalized salinity readings are supposed to be between 0.0 and 1.0. Write a query that selects all records from
Surveywith salinity values outside this range.
Which of these expressions are true?
'a' LIKE 'a'
'a' LIKE '%a'
'beta' LIKE '%a'
'alpha' LIKE 'a%%'
'alpha' LIKE 'a%p%'
Use WHERE to specify conditions that records must meet in order to be included in a query’s results.
Use AND, OR, and NOT to combine tests.
Filtering is done on whole records, so conditions can use fields that are not actually displayed.
Write queries incrementally.