SQL Aggregation

Overview

Teaching: 30 min
Exercises: 5 min

Questions

• How can I summarize my data by aggregating, filtering, or ordering query results?

Objectives

• Apply aggregation to group records in SQL.

• Filter and order results of a query based on aggregate functions.

• Save a query to make a new table.

• Apply filters to find missing values in SQL.

COUNT and GROUP BY

Aggregation allows us to combine results by grouping records based on value and calculating combined values in groups.

Let’s go to the surveys table and find out how many individuals there are. Using the wildcard simply counts the number of records (rows):

SELECT COUNT(*)
FROM surveys;

We can also find out how much all of those individuals weigh:

SELECT COUNT(*), SUM(weight)
FROM surveys;

We can output this value in kilograms, rounded to 3 decimal places:

SELECT ROUND(SUM(weight)/1000, 3)
FROM surveys;

There are many other aggregate functions included in SQL including MAX, MIN, and AVG.

Challenge

Write a query that returns: total weight, average weight, and the min and max weights for all animals caught over the duration of the survey. Can you modify it so that it outputs these values only for weights between 5 and 10?

Now, let’s see how many individuals were counted in each species. We do this using a GROUP BY clause

SELECT species_id, COUNT(*)
FROM surveys
GROUP BY species_id;

GROUP BY tells SQL what field or fields we want to use to aggregate the data. If we want to group by multiple fields, we give GROUP BY a comma separated list.

Challenge

Write queries that return:

1. How many individuals were counted in each year
   • in total
   • per each species
2. Average weight of each species in each year.

Can you modify the above queries combining them into one?

The HAVING keyword

In the previous lesson, we have seen the keywords WHERE, allowing to filter the results according to some criteria. SQL offers a mechanism to filter the results based on aggregate functions, through the HAVING keyword.

For example, we can adapt the last request we wrote to only return information about species with a count higher than 10:

SELECT species_id, COUNT(species_id)
FROM surveys
GROUP BY species_id
HAVING COUNT(species_id) > 10;

The HAVING keyword works exactly like the WHERE keyword, but uses aggregate functions instead of database fields.

If you use AS in your query to rename a column, HAVING can use this information to make the query more readable. For example, in the above query, we can call the COUNT(species_id) by another name, like occurrences. This can be written this way:

SELECT species_id, COUNT(species_id) AS occurrences
FROM surveys
GROUP BY species_id
HAVING occurrences > 10;

Note that in both queries, HAVING comes after GROUP BY. One way to think about this is: the data are retrieved (SELECT), can be filtered (WHERE), then joined in groups (GROUP BY); finally, we only select some of these groups (HAVING).

Challenge

Write a query that returns, from the species table, the number of genus in each taxa, only for the taxa with more than 10 genus.

Ordering Aggregated Results

We can order the results of our aggregation by a specific column, including the aggregated column. Let’s count the number of individuals of each species captured, ordered by the count:

SELECT species_id, COUNT(*)
FROM surveys
GROUP BY species_id
ORDER BY COUNT(species_id);

Saving Queries for Future Use

It is not uncommon to repeat the same operation more than once, for example for monitoring or reporting purposes. SQL comes with a very powerful mechanism to do this: views. Views are a form of query that is saved in the database, and can be used to look at, filter, and even update information. One way to think of views is as a table, that can read, aggregate, and filter information from several places before showing it to you.

Creating a view from a query requires to add CREATE VIEW viewname AS before the query itself. For example, imagine that my project only covers the data gathered during the summer (May - September) of 2000. That query would look like:

SELECT *
FROM surveys
WHERE year = 2000 AND (month > 4 AND month < 10);

But we don’t want to have to type that every time we want to ask a question about that particular subset of data. Let’s create a view:

CREATE VIEW summer_2000 AS
SELECT *
FROM surveys
WHERE year = 2000 AND (month > 4 AND month < 10);

You can also add a view using Create View in the View menu and see the results in the Views tab just like a table.

Now, we will be able to access these results with a much shorter notation:

SELECT *
FROM summer_2000;

There should only be six records. If you look at the weight column, it’s easy to see what the average weight would be. If we use SQL to find the average weight, SQL behaves like we would hope, ignoring the NULL values:

SELECT AVG(weight)
FROM summer_2000
WHERE species_id == 'PE';

But if we try to be extra clever, and find the average ourselves, we might get tripped up:

SELECT SUM(weight), COUNT(*), SUM(weight)/COUNT(*)
FROM summer_2000
WHERE species_id == 'PE';

Here the COUNT command includes all six records (even those with NULL values), but the SUM only includes the 4 records with data in the weight field, giving us an incorrect average. However, our strategy will work if we modify the count command slightly:

SELECT SUM(weight), COUNT(weight), SUM(weight)/COUNT(weight)
FROM summer_2000
WHERE species_id == 'PE';

When we count the weight field specifically, SQL ignores the records with data missing in that field. So here is one example where NULLs can be tricky: COUNT(*) and COUNT(field) can return different values.

Another case is when we use a “negative” query. Let’s count all the non-female animals:

SELECT COUNT(*)
FROM summer_2000
WHERE sex != 'F';

Now let’s count all the non-male animals:

SELECT COUNT(*)
FROM summer_2000
WHERE sex != 'M';

But if we compare those two numbers with the total:

SELECT COUNT(*)
FROM summer_2000;

We’ll see that they don’t add up to the total! That’s because SQL doesn’t automatically include NULL values in a negative conditional statement. So if we are quering “not x”, then SQL divides our data into three categories: ‘x’, ‘not NULL, not x’ and NULL and returns the ‘not NULL, not x’ group. Sometimes this may be what we want - but sometimes we may want the missing values included as well! In that case, we’d need to change our query to:

SELECT COUNT(*)
FROM summer_2000
WHERE sex != 'M' OR sex IS NULL;

There is one more subtlety we need to be aware of. Suppose we run this query:

SELECT COUNT(*), weight
FROM summer_2000;

Key Points

• Use the GROUP BY keyword to aggregate data.

• Functions like MIN, MAX, AVERAGE, SUM, COUNT, etc. operate on aggregated data.

• Use the HAVING keyword to filter on aggregate properties.

• Use a VIEW to access the result of a query as though it was a new table.