The videos from pgDay Paris 2019 are online, and they look stunning! If you’ve been to the conference, you will be able to enjoy great memories, and if you’ve not been there, you can now enjoy all the fine content that was recorded there at the Youtube Channel for pgDay.Paris.

This year at pgday Paris I tried something new. My presentation was interactive, and I’ve been writing queries together with the audience, making mistakes, loosing keystrokes and having all the fun!

The idea behind looking like a fool writing queries in front of an audience was to show the process behind those elegant queries that we tend to share in one piece, on a slide where they shine, beautifully indented. We don’t get to those queries writing them like we would prose. It takes multiple iterations to get there. From something quite simple to something more and more sophisticated, until we have exactly the result set we are interested in.

This article is a transcript of the conference I gave at Postgres Open 2019, titled the same as the book: The Art of PostgreSQL. It’s availble as a video online at Youtube if you want to watch the slides and listen to it, and it even has a subtext!

This article is a transcript of the conference I gave at Postgres Open 2019, titled the same as the book: The Art of PostgreSQL. It’s availble as a video online at Youtube if you want to watch the slides and listen to it, and it even has a subtext!

This article is a transcript of the conference I gave at Postgres Open 2019, titled the same as the book: The Art of PostgreSQL. It’s availble as a video online at Youtube if you want to watch the slides and listen to it, and it even has a subtext!

Here we are, another SQL query to write. You wish you knew how to write that mechanically, like you would a loop in your favorite programming language. Or at least have a pretty clear idea of a skeleton to tweak until it gives the result set you expect. So instead of working on your SQL query, you google How to write a SQL query? or maybe even How to learn SQL? Right. I feel you, I’ve been there too, even if quite some time ago…

So here my article where I teach you how to learn SQL.

I want to share with you how I did it, and how I continue to do it. There’s no magic secret sauce to it though, it’s all basic work. Again, we have to learn the main concepts and how they play together, then practice simple steps, and then build from there.

Ok, let’s face it, I like SQL. A lot. I think it’s a fine DSL given how powerful it is, and I respect its initial goal to attract non developers and try to build English sentences rather than code.

Also, I understand that manually hydrating your collection of objects in your backend developement language is not the best use of your time. And that building SQL as strings makes your code ugly. I get it.

In our previous articles we had a look at What is an SQL relation? and What is a SQL JOIN?. Today, I want to show you what is an aggregate in SQL.

You might have heard about Map/Reduce when Google made it famous, or maybe some years before, or maybe some years later. The general processing functions map and reduce where invented a very long time ago. The novelty from the advertising giant was in using them in a heavily distributed programming context.

With map you apply the same processing to every object in a collection. With reduce, you compute a result per collection of object. In SQL we know how to do that too, and we call that an aggregate.

It took me quite some time before I could reason efficiently about SQL JOINs. And I must admit, the set theory diagrams never helped me really understand JOINs in SQL. So today, I want to help you understand JOINs in a different way, hoping to make the concept click at once for you too!

If you’re like me, understanding SQL is taking you a long time. And some efforts. It took me years to get a good mental model of how SQL queries are implemented, and then up from the lower levels, to build a mental model of how to reason in SQL.

Nowadays, in most case, I can think in terms of SQL and write queries that will give me the result set I need. In most cases, because some parts of SQL are still complex, and I don’t practice as much as I used to. In particular, newer PostgreSQL features are areas I still need to work with more before I can integrate them in my daily workflow.

Today, I want to introduce the central concept of the SQL language. What is an SQL relation?