Accessing Data With Queries
OverviewTeaching: 30 min
Exercises: 5 minQuestions
How do I write a basic query in SQL?Objectives
Write and build queries.
Filter data given various criteria.
Sort the results of a query.
Writing my first query
Let’s start by using the surveys table. Here we have data on every individual that was captured at the site, including when they were captured, what plot they were captured on, their species ID, sex and weight in grams.
Let’s write an SQL query that selects all of the columns in the surveys table. SQL queries can be written in the box located under the “Execute SQL” tab. Click on the right arrow above the query box to execute the query. (You can also use the keyboard shortcut “Cmd-Enter” on a Mac or “Ctrl-Enter” on a Windows machine to execute a query.) The results are displayed in the box below your query. If you want to display all of the columns in a table, use the wildcard *.
SELECT * FROM surveys;
We have capitalized the words SELECT and FROM because they are SQL keywords. SQL is case insensitive, but it helps for readability, and is good style.
If we want to select a single column, we can type the column name instead of the wildcard *.
SELECT year FROM surveys;
If we want more information, we can add more columns to the list of fields, right after SELECT:
SELECT year, month, day FROM surveys;
Sometimes you don’t want to see all the results, you just want to get a sense of what’s being returned. In that case, you can use the
LIMIT command. In particular, you would want to do this if you were working with large databases.
SELECT * FROM surveys LIMIT 10;
If we want only the unique values so that we can quickly see what species have
been sampled we use
SELECT DISTINCT species_id FROM surveys;
If we select more than one column, then the distinct pairs of values are returned
SELECT DISTINCT year, species_id FROM surveys;
We can also do calculations with the values in a query. For example, if we wanted to look at the mass of each individual on different dates, but we needed it in kg instead of g we would use
SELECT year, month, day, weight/1000 FROM surveys;
When we run the query, the expression
weight / 1000 is evaluated for each
row and appended to that row, in a new column. If we used the
INTEGER data type
for the weight field then integer division would have been done, to obtain the
correct results in that case divide by
1000.0. Expressions can use any fields,
any arithmetic operators (
/) and a variety of built-in
functions. For example, we could round the values to make them easier to read.
SELECT plot_id, species_id, sex, weight, ROUND(weight / 1000, 2) FROM surveys;
- Write a query that returns the year, month, day, species_id and weight in mg.
SELECT day, month, year, species_id, weight * 1000 FROM surveys;
Databases can also filter data – selecting only the data meeting certain
criteria. For example, let’s say we only want data for the species
Dipodomys merriami, which has a species code of DM. We need to add a
WHERE clause to our query:
SELECT * FROM surveys WHERE species_id='DM';
We can do the same thing with numbers. Here, we only want the data since 2000:
SELECT * FROM surveys WHERE year >= 2000;
If we used the
TEXT data type for the year, the
WHERE clause should
year >= '2000'.
We can use more sophisticated conditions by combining tests
OR. For example, suppose we want the data on Dipodomys merriami
starting in the year 2000:
SELECT * FROM surveys WHERE (year >= 2000) AND (species_id = 'DM');
Note that the parentheses are not needed, but again, they help with
readability. They also ensure that the computer combines
in the way that we intend.
If we wanted to get data for any of the Dipodomys species, which have
DS, we could combine the tests using OR:
SELECT * FROM surveys WHERE (species_id = 'DM') OR (species_id = 'DO') OR (species_id = 'DS');
- Produce a table listing the data for all individuals in Plot 1 that weighed more than 75 grams, telling us the date, species id code, and weight (in kg).
SELECT day, month, year, species_id, weight / 1000 FROM surveys WHERE (plot_id = 1) AND (weight > 75);
Building more complex queries
Now, let’s combine the above queries to get data for the 3 Dipodomys species from
the year 2000 on. This time, let’s use IN as one way to make the query easier
to understand. It is equivalent to saying
WHERE (species_id = 'DM') OR (species_id
= 'DO') OR (species_id = 'DS'), but reads more neatly:
SELECT * FROM surveys WHERE (year >= 2000) AND (species_id IN ('DM', 'DO', 'DS'));
We started with something simple, then added more clauses one by one, testing their effects as we went along. For complex queries, this is a good strategy, to make sure you are getting what you want. Sometimes it might help to take a subset of the data that you can easily see in a temporary database to practice your queries on before working on a larger or more complicated database.
When the queries become more complex, it can be useful to add comments. In SQL,
comments are started by
--, and end at the end of the line. For example, a
commented version of the above query can be written as:
-- Get post 2000 data on Dipodomys' species -- These are in the surveys table, and we are interested in all columns SELECT * FROM surveys -- Sampling year is in the column `year`, and we want to include 2000 WHERE (year >= 2000) -- Dipodomys' species have the `species_id` DM, DO, and DS AND (species_id IN ('DM', 'DO', 'DS'));
Although SQL queries often read like plain English, it is always useful to add comments; this is especially true of more complex queries.
We can also sort the results of our queries by using
For simplicity, let’s go back to the species table and alphabetize it by taxa.
First, let’s look at what’s in the species table. It’s a table of the species_id and the full genus, species and taxa information for each species_id. Having this in a separate table is nice, because we didn’t need to include all this information in our main surveys table.
SELECT * FROM species;
Now let’s order it by taxa.
SELECT * FROM species ORDER BY taxa ASC;
ASC tells us to order it in ascending order.
We could alternately use
DESC to get descending order.
SELECT * FROM species ORDER BY taxa DESC;
ASC is the default.
We can also sort on several fields at once. To truly be alphabetical, we might want to order by genus then species.
SELECT * FROM species ORDER BY genus ASC, species ASC;
- Write a query that returns year, species_id, and weight in kg from the surveys table, sorted with the largest weights at the top.
SELECT year, species_id, weight / 1000 FROM surveys ORDER BY weight DESC;
Order of execution
Another note for ordering. We don’t actually have to display a column to sort by it. For example, let’s say we want to order the birds by their species ID, but we only want to see genus and species.
SELECT genus, species FROM species WHERE taxa = 'Bird' ORDER BY species_id ASC;
We can do this because sorting occurs earlier in the computational pipeline than field selection.
The computer is basically doing this:
- Filtering rows according to WHERE
- Sorting results according to ORDER BY
- Displaying requested columns or expressions.
Clauses are written in a fixed order:
BY. It is possible to write a query as a single line, but for readability,
we recommend to put each clause on its own line.
- Let’s try to combine what we’ve learned so far in a single query. Using the surveys table, write a query to display the three date fields,
species_id, and weight in kilograms (rounded to two decimal places), for individuals captured in 1999, ordered alphabetically by the
- Write the query as a single line, then put each clause on its own line, and see how more legible the query becomes!
SELECT year, month, day, species_id, ROUND(weight / 1000, 2) FROM surveys WHERE year = 1999 ORDER BY species_id;
It is useful to apply conventions when writing SQL queries to aid readability.
Use logical connectors such as AND or OR to create more complex queries.
Calculations using mathematical symbols can also be performed on SQL queries.
Adding comments in SQL helps keep complex queries understandable.