SQL GROUP BY: Everything you need to know about this query

One of the most useful SQL commands for data analysts is GROUP BY. It allows you to easily generate advanced statistics from categories. Suffice to say, it’s one of the 10 commands you’ll use the most, so you might as well know what it’s all about.

SQL’s strengths are well established. This language offers an unrivalled level of abstraction for analyzing vast volumes of information. Whatever the database software used (MySQL, Ingres, Oracle Database, Microsoft SQL Server, PostgreSQL, SQLite, Firebird…), the data analyst knows he’ll get the desired result without having to program the precise logic.

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How do you group information in a database?

If there’s one particularly important command you’ll appreciate, it’s SQL GROUP BY. It groups information from a database according to a particular column, from which it is possible to obtain statistical information: sum, maximum, minimum, average, etc.

To better understand this concept, it’s best to start with an example. Here’s an extract from a table called Staff

If we wanted to obtain the sum of salaries by department, we’d simply use a sequence like the following:

SELECT Department, SUM(Salary)

FROM Staff

GROUP BY Service

ORDER BY Service;

In this example, we group the table according to the Service column and ask for the sum of salaries for each of them.

The answer would be – based on the sample shown above :

How do I use the HAVING BY clause?

HAVING BY completes the SQL GROUP BY command by opening up the possibility of specifying a condition. In the example below, we’d like to display the average salary (the AVG function), but only if this average is greater than 2500:

SELECT Department, AVG(Salary)

FROM Personnel

GROUP BY Service

HAVING AVG(Salary) > 2500

ORDER BY Service;

The result is :

And that’s it! It’s that simple. The ease with which such operations can be implemented is a testament to the qualities of the SQL language.

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