The love-hate relationship between SELECT and GROUP BY in SQL

I was recently working on some database queries (in PostgreSQL) and encountered the following error:

I felt ashamed not being able to explain in a clear manner why that happened. Especially, when a colleague reminded me that in MySQL that works (scroll at the bottom to read more about it).

They say the best way to understand anything is teaching it, so here I am. But first let’s backtrack a bit.

Let’s say we have two tables: films and reviews with the following schemas:

  • films -> id (int), title (string), genre (string);
  • reviews -> id (int), film_in (int), content (string).

Also, films contains the following records:

And reviews :

Let’s say we want to count how many movies of one specific genre are. This is where GROUP BY comes handy. In fact, it allows grouping the result-set by one or more columns and calculate aggregate functions on the grouped records:

In the previous query, count indicates how many records with the same value in the column genre are in films: two thrillers (i.e. Blade Runner 2049 and Inception) and one scifi (i.e. Arrival). In other words, count represents how many records were grouped together by genre.

Let’s say we wanted to know which titles belong to each genre group. We could try with:

Except we cannot. In fact, there’s no way for the database engine to select a title since there could be more than one record with the same genre. Problem that is actually present in our example: there are two thrillers.

Well, we could blindly add the title column to the GROUB BY as the first part of the error suggest, correct?

Wrong, this is not the query we want to execute.

It’s the second part of the error what we are looking for (i.e. or be used in an aggregate function). In fact, there’s a set of aggregate functions that databases offer us. In PostgreSQL count() is one example we’ve already seen, but there’s a lot more.

In our case, string_agg() is a good choice:

Up until now we have seen that in the SELECT statement we can have only columns that either appear in the GROUP BY or are arguments of an aggregate function.

Well, this is not the whole truth. In Roland Bouman’s words “the 1999 and 2003 versions of the SQL standard require that the columns appearing in the SELECT list are functionally dependent upon the groups defined by the GROUP BY clause. In other words, if we know that a column contains only one value for any given combination of values in the columns appearing in the GROUP BY clause, we may reference the column in the SELECT list even if it does not appear in an aggregate expression”.

For example, a primary or unique key satisfies that rule. In fact, by selecting one value of a unique key, the value of the other columns have only one combination of values. In other words, there’s only one record with that specific value in the unique key. Or better yet, the values in the other columns are functionally dependent upon the unique key.

Therefore, since id is a primary key (i.e. unique by definition) all other columns in the same table are functionally dependent upon it. That’s why the following query does not raise any errors:

Of course, grouping by a unique column when querying one table does not make any sense. But it comes useful when other tables are involved:

It’s interesting to notice that MySQL didn’t historically conform to SQL standards. As a matter of fact, the engine let you SELECT anything in a query with a GROUP BY . Including non-aggregated columns that do not appear in the GROUP BY and that are not functionally dependent upon the GROUP BY clause:

As you can see, the engine picked a random record out of the thriller group. This is concerning and I believe databases should at least warn users in that case.

Luckily, the correct behaviour can be enabled by adding a flag (i.e. ONLY_FULL_GROUP_BY) or by using MySQL v5.7.5 or higher. In that case, the query would raise the following error:

All you need to know can be found in this in-depth post by Roland Bouman: Debunking GROUP BY myths.

This post is dedicated to the best code concierge out there: Tomek Rusiłko. The one who keeps inspiring me with great food for thoughts and pushed me to do more of “learn by teaching”. Check him out if you are up for some great conference talks related to programming.

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