5

A Small Application

Let’s write a very basic application where we’re going to compare using either classic application code or SQL to solve some common problems. Our goal in this section is to be confronted with managing SQL as part of a code base, and show when to use classic application code or SQL.

Readme First Driven Development

Before writing any code or tests or anything, I like to write the readme rst. That’s this little le explaining to the user why to care for about the application, and maybe some details about how to use it. Let’s do that now.

The cdstore application is a very simple wrapper on top of the Chinook database. The Chinook data model represents a digital media store, including tables for artists, albums, media tracks, invoices, and customers.

The cdstore application allows listing useful information and reports on top of the database, and also provides a way to generate some activity.

Chapter 5 A Small Application j 42

Loading the Dataset

When I used the Chinook dataset rst, it didn’t support PostgreSQL, so I used theSQLitedataoutput, whichnicely tsintoasmallenoughdata le. Nowadays you will nd a PostgreSQL backup le that you can use. It’s easier for me to just use pgloader though, so I will just do that.

Another advantage of using pgloader in this book is that we have the following summary output, which lists tables and how many rows we loaded for each of them. This is the rst encounter with our dataset.

Here’s a truncated output from the pgloader run (edited so that it can t in the book page format):

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"idx_51519_ifk_trackalbumid" btree (albumid) "idx_51519_ifk_trackgenreid" btree (genreid) "idx_51519_ifk_trackmediatypeid" btree (mediatypeid)

... foreign keys ...

> alter table track add primary key using index idx_51519_ipk_track; ALTER TABLE

Note that as PostgreSQL implements group by inference we need this primary key to exists in order to be able to run some of the following queries. This means that as soon as you’ve loaded the dataset, please x the primary key so that we are ready to play with the dataset.

Chinook Database

The Chinook database includes basic music elements such as album, artist, track, genre and mediatype for a music collection. Also, we nd the idea of a playlist with an association table playlisttrack, because any track can take part of several playlists and a single playlist is obviously made of several tracks.

Then there’s a model for a customer paying for some tracks with the tables staff, customer, invoice and invoiceline.

pgloader# \dt chinook.

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Chapter 5 A Small Application j 44

chinook │ playlisttrack │ table │ dim

(11 rows)

With that in mind we can begin to explore the dataset with a simple query:

1select genre.name, count(*) as count

4group by genre.name

5order by count desc;

Which gives us:

name │ count

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(25 rows)