Aggregation

Overview

Teaching: 30 min
Exercises: 30 min
Questions
  • How do you aggregate records in SQL?

Objectives
  • to understand how to aggregate records 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 articles table and find out how many entries there are. Using the wildcard simply counts the number of records (rows)

SELECT COUNT(*)
FROM articles;

We can also find out how many authors have participated in these articles.

SELECT COUNT(*), SUM(author_count)
FROM articles;

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

Challenge

Write a query that returns the total, average, minimum and maximum number of citations of all of the articles.

Solution

SELECT SUM(citation_count), AVG(citation_count), MIN(citation_count), MAX(citation_count)
FROM articles;

Now, let’s see how many articles were published in each journal. We do this using a GROUP BY clause

SELECT issns, COUNT( * )
FROM articles
GROUP BY issns;

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 citations that were counted each month a) in total; b) per journal.
  2. Average number of citations of each journal in each month.
  3. Can you modify the above queries combining them into one?

Solution 1a)

SELECT month, SUM(citation_count)
FROM articles
GROUP BY month;

Solution 1b)

SELECT month, issns, SUM(citation_count)
FROM articles
GROUP BY month, issns;

Solution 2)

SELECT month, issns, AVG(citation_count)
FROM articles
GROUP BY month, issns;

Solution 3)

SELECT month, issns, AVG(citation_count), SUM(citation_count)
FROM articles
GROUP BY month, issns;

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 journals with 10 or more published articles:

SELECT issns, COUNT( * )
FROM articles
GROUP BY issns
HAVING COUNT( * ) >= 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(*) by another name, like occurrences. This can be written this way:

SELECT issns, COUNT( * ) AS occurrences
FROM articles
GROUP BY issns
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 articles table, the average number of citations for each journal, only for the journals with 5 or more citations on average.

Solution

SELECT issns, AVG(citation_count)
FROM articles
GROUP BY issns
HAVING AVG(citation_count)>=5;

Challenge

Write a query that returns, from the articles table: the total, average, min and max number of citations for each journal. Can you modify it so that it outputs only journals with more than 5 citations in average?

Solution

SELECT issns,  SUM(citation_count), AVG(citation_count), MIN(citation_count), MAX(citation_count)
FROM articles
GROUP BY issns
HAVING AVG(citation_count)>5;

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 articles published in each journal, ordered by the count

SELECT issns, COUNT( * )
FROM articles
GROUP BY issns
ORDER BY COUNT( * ) DESC;

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 queries saved in the database. You query it as a (virtual) table, populated every time you query it.

Creating a view from a query requires to add CREATE VIEW viewname AS before the query itself. For example, if we want to save the query giving the number of journals in a view, we can write

CREATE VIEW journal_counts AS
SELECT issns, COUNT(*)
FROM articles
GROUP BY issns;

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

SELECT *
FROM journal_counts;

Assuming we do not need this view anymore, we can remove it from the database almost as we would a table:

DROP VIEW journal_counts;

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

Challenge

Write a query that returns the number of articles published in each journal on each month, sorted from most popular journal to the ones with least publications each month starting from the most recent records. Save this query as a VIEW.

Solution

CREATE VIEW journal_counts AS
SELECT COUNT(*), month
FROM articles
GROUP BY issns, month
ORDER BY  count(*) desc, month desc;

Key Points

  • SQL is ideal for aggregating database records