Aggregating & calculating values

Overview

Teaching: 15 min
Exercises: 5 min
Questions
  • How can we aggregate values in SQL for reports?

  • Can SQL be used to make calculations?

Objectives
  • Use SQL functions like AVG in combination with clauses like Group By to aggregate values and return results for reports.

  • Make calculations on fields using SQL.

Aggregation

SQL contains functions which allow you to make calculations on data in your database for reports. Some of the most common functions are MAX, MIN, AVG, COUNT, SUM, and they will: MAX (find the maximum value in a field), MIN (find the minimum value in a field), AVG (find the average value of a field), COUNT (count the number of values in a field and present the total), and SUM (add up the values in a field and present the sum).

Let’s say we wanted to get the average Citation_Count for each of the ISSNs. We can use AVG and the GROUP BY clause in a query:

SELECT ISSNs, AVG(Citation_Count)
FROM articles
GROUP BY ISSNs;

GROUP BY is used by SQL to arrange identical data into groups. In this case, we are arranging all the citation counts by ISSNs. AVG acts on the Citation_Count in parentheses. This process is also called aggregation which allows us to combine results by grouping records based on value and calculating combined values in groups.

As you can see, it is difficult to tell though what ISSN has the highest average citation count and the least. We can improve upon the query above by using ORDER BY and DESC.

SELECT ISSNs, AVG(Citation_Count)
FROM articles
GROUP BY ISSNs 
ORDER BY Avg_Citation_Count DESC;

Challenge

Write a query using an aggregate function that returns the number of article titles per ISSNs, sorted by title count in descending order. Which ISSN has the most titles? (Hint to choosing which aggregate function to use - it is one of the common aggreggate functions MAX, MIN, AVG, COUNT, SUM.)

Solution

SELECT  ISSNs, COUNT(Title)
FROM articles
GROUP BY ISSNs
ORDER BY Title_Count DESC;

The HAVING keyword

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 journal ISSNs with 10 or more published articles:

SELECT ISSNs, COUNT(*)
FROM articles
GROUP BY ISSNs
HAVING Record_Count >= 10;

The HAVING keyword works exactly like the WHERE keyword, but uses aggregate functions instead of database fields. When you want to filter based on an aggregation like MAX, MIN, AVG, COUNT, SUM, use HAVING; to filter based on the individual values in a database field, use WHERE.

Note that 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 Citation_Count for each journal ISSN but 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;

Calculations

In SQL, we can also perform calculations as we query the database. Also known as computed columns, we can use expressions on a column or multiple columns to get new values during our query. For example, what if we wanted to calculate a new column called CoAuthor_Count:

SELECT Title, ISSNs, Author_Count -1
FROM articles
ORDER BY Author_Count -1 DESC;

We can use any arithmetic operators (+, -, *, and /) if we would like.

If you would like to learn more about calculated values, the Software Carpentry Databases and SQL lesson includes a useful episode on Calculating New Values.

Key Points

  • SQL can be used for reporting purposes.

  • Queries can do arithmetic operations on field values.