dbt for Data Transformation - A Hands-on Tutorial
dbt for Data Transformation - A Hands-on Tutorial

dbt for Data Transformation - A Hands-on Tutorial

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9 min read
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DatabaseGuide toSQLData TransformationETLData ModelingData ScienceTutorial
Last Updated
Jul 17, 2021
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Use dbt for complex data transformations by writing SQL SELECT statements only. Run tests, generate documentation, use macros, and many more features.
Table of Content

dbt (data build tool) is a data transformation tool that uses select SQL statements. It allows you to create complex models, use variables and macros (aka functions), run tests, generate documentation, and many more features.

dbt does not extract or load data, but it’s powerful at transforming data that’s already available in the database —dbt does the T in ELT (Extract, Load, Transform) processes.

In this post, you will learn how to ...

  • Configuring a dbt project
  • Creating dbt models (SELECT statements)
  • Build complex dbt models using global variables and macros
  • Building complex models by referring to other dbt models
  • Running tests
  • Generating documentation

Pre-requisite

Signup

You can sign up at getdbt.com. The free plan is a great plan for small projects and testing.

Database with populated data

You can check my post on how to deploy a free PostgreSQL database on Heroku. The post provides step-by-step instructions on how to do it.

You can also check the data ingestion script in the GitHub repo accompanying this article.

Following the above, we have generated two tables in a PostgreSQL database that we are going to use in this post. There are two tables in the database, namely covid_latest and population_prosperity. You can find the ingestion script on the GitHub repo for this post.

dbt CLI Installation

You can install the dbt command-line interface (CLI) by following the instructions on the following dbt documentation page.

Basics of a dbt project

There are three main things to know about in order to use the dbt tool.

  • dbt project
  • database connection
  • dbt commands

How to use dbt?

A dbt project is a directory containing .sql and .yml files. The minimum required files are:

  • A project file named dbt_project.yml: This file contains configurations of a dbt project.
  • Model(s) (.sql files): A model in dbt is simply a single .sql file containing a single select statement.

Every dbt project needs a dbt_project.yml file — this is how dbt knows a directory is a dbt project. It also contains important information that tells dbt how to operate on your project.

You can find more information about dbt projects here.

💡

A dbt model is basically a .sql file with a SELECT statement.

dbt Commands

dbt commands start with dbt and can be executed using one of the following ways:

  • dbt Cloud (the command section at the bottom of the dbt Cloud dashboard),
  • dbt CLI

Some commands can only be used in dbt CLI like dbt init. Some dbt commands we will use in this post are

  • dbt init (only in dbt CLI)
  • dbt run
  • dbt test
  • dbt docs generate

dbt Project Setup

Step 1: Initialize a dbt project (sample files) using dbt CLI

You can use dbt init to generate sample files/folders. In particular, dbt init project_name will create the following:

  • ~/.dbt/profiles.yml file if one does not already exist
  • a new folder called [project_name]
  • directories and sample files necessary to get started with dbt

⚠️

Caution: Since dbt init generates a directory namedproject_name, and in order to avoid any conflict, you should not have any existing folder with an identical name.

dbt init <project_name>
dbt init <project_name>

The result is a directory with the following sample files.

sample_dbt_project
├── README.md
├── analysis
├── data
├── dbt_project.yml
├── macros
├── models
│   └── example
│       ├── my_first_dbt_model.sql
│       ├── my_second_dbt_model.sql
│       └── schema.yml
├── snapshots
└── tests

For this post, we will just consider the minimum files and remove the extra stuff.

sample_dbt_project
├── README.md
├── dbt_project.yml
└── models
    ├── my_first_dbt_model.sql
    ├── my_second_dbt_model.sql
    └── schema.yml

Step 2: Set Up a Git Repository

You can use an existing repo, as specified during the setup. You can configure the repositories by following the dbt documentation here.

Or, if you want to create a new repo...

you can create a new repository from inside the created directory. You can do that as below

git init
git add .
git commit -m "first commit"
git remote add origing <repo_url>
git push -u origin master

Step 3: Set Up a New Project on dbt Cloud Dashboard

In the previous step, we created a sample dbt project containing sample models and configurations. Now, we want to create a new project and connect our database and repository on the dbt Cloud dashboard.

Before we continue, you should have

  • some data already available in a database,
  • a repository with the files generated at the previous step

You can follow the steps below to set up a new project in dbt Cloud (keep in mind this step is different than the previous step in that we only generated some sample files).

Set up a new dbt project on dbt Cloud

The dbt_project.yml file for our project is shown below (you can find the complete version in the GitHub repo to this post).

name: 'my_new_project'
version: '1.0.0'
config-version: 2

vars:
  selected_country: USA
	selected_year: 2019

# This setting configures which "profile" dbt uses for this project.
profile: 'default'

# There are other stuff that are generated automatically when you run `dbt init`
dbt_project.yml

dbt Models and Features

dbt models

Let's create simple dbt models that retrieve few columns of the tables.

select "iso_code", "total_cases", "new_cases" from covid_latest
covid19_latest_stats dbt model (models/covid19_latest_stats.sql)
select "code", "year", "continent", "total_population" from population_prosperity
population dbt model (models/population.sql)
⚠️

Caution: The dbt model name is the filename of the sql file in the models directory. The model name may differ from the table name in the database. For instance, in above, the dbt model population is the result of a SELECT statement on population_prosperity table in the database.

Run models

You can run all models in your dbt project by executing dbt run. A sample dbt run output is shown below. You can see a summary or detailed log of running all dbt models. This helps a lot to debug any issue you may have in the queries. For instance, you can see a failed model that throws a Postgres error.

Detailed log of failed
Detailed log of failed jinja_and_variable_usage dbt model

Jinja & Macros

dbt uses Jinja templating language, making a dbt project an ideal programming environment for SQL. With Jinja, you can do transformations that are not normally possible in SQL, like using environment variables, or macros — abstract snippets of SQL, which is analogous to functions in most programming languages. Whenever you see a {{ ... }}, you're already using Jinja. For more information about Jinja and additional Jinja-style functions defined, you can check dbt documentation.

Later in this post, we will cover custom macros defined by dbt.

Using Variables

Define a variable

You can define your variables under the vars section in your dbt_project.yml. For instance, let's define a variable called selected_country whose default value is USA and another one called selected_year whose default value is 2019.

name: 'my_new_project'
version: '1.0.0'
config-version: 2

vars:
  selected_country: USA
	selected_year: 2019
dbt_project.yml

Use a Variable

You can use variables in your dbt models via var() Jinja function ({{ var("var_key_name") }} .

Macros

There are many useful transformations and useful macros in dbt_utils that can be used in your project. For a list of all available macros, you can check their GitHub repo.

Now, let's add dbt_utils to our project and install it by following the below steps:

  1. Add dbt_utils macro to your packages.yml file, as follows:
packages:
  - package: dbt-labs/dbt_utils
    version: 0.6.6
Add dbt_utils package to packages.yml

2. Run dbt deps to install the package.

Install packages using
Install packages using dbt deps

Complex dbt models

The models (selects) are usually stacked on top of one another. For building more complex models, you will have to use ref() macro. ref() is the most important function in dbt as it allows you to refer to other models. For instance, you may have a model (aka SELECT query) that does multiple stuff, and you don’t want to use it in other models. It will be difficult to build a complex model without using macros introduced earlier.

dbt model using ref() and global variables

We can build more complex models using the two dbt models defined earlier in the post. For instance, let's create a new dbt model that joins the above two tables on the country code and then filters based on selected country and year.

select *
from {{ref('population')}} 
inner join {{ref('covid19_latest_stats')}} 
on {{ref('population')}}.code = {{ref('covid19_latest_stats')}}.iso_code 
where code='{{ var("selected_country") }}' AND year='{{ var("selected_year") }}'
jinja_and_variable_usage dbt model (models/jinja_and_variable_usage.sql)

Few points about the query above:

  • {{ref('dbt_model_name')}}is used to refer to dbt models available in the project.
  • You can get a column from the model like {{ref('dbt_model_name')}}.column_name.
  • You can use variables defined in dbt_project.yml file by {{var("variable_name)}}.

The abbove code snippet joins the data from population and covid19_latest_stats models on the country code and filters them based on the selected_country=USA and selected_year=2019. The output of the model is shown below.

The output of the
The output of the jinja_and_variable_usage dbt model

You can also see the compiled SQL code snippet by clicking on compile sql button. This is very useful particularly if you want to run the query outside the dbt tool.

Compiled SQL code for
Compiled SQL code for jinja_and_variable_usage dbt model

dbt model using dbt_utils package and macros

dbt_utils package contains macros (aka functions) you can use in your dbt projects. A list of all macros is available on dbt_utils' GitHub page.

Let's use dbt_utils pivot() and get_column_values() macros in a dbt model as below:

select
  continent,
  {{ dbt_utils.pivot(
      "population.year",
      dbt_utils.get_column_values(ref('population'), "year")
  ) }}
from {{ ref('population') }}
group by continent
using_dbt_utils_macros dbt model (models/using_dbt_utils_macros.sql)

The above dbt model will compile to the following SQL query in dbt.

select
  continent,
    sum(case when population.year = '2015' then 1 else 0 end) as "2015",
		sum(case when population.year = '2017' then 1 else 0 end) as "2017",
		sum(case when population.year = '2017' then 1 else 0 end) as "2016",
		sum(case when population.year = '2017' then 1 else 0 end) as "2018",
		sum(case when population.year = '2017' then 1 else 0 end) as "2019"
from "d15em1n30ihttu"."dbt_ealizadeh"."population"
group by continent
limit 500
/* limit added automatically by dbt cloud */
Compiled SQL query from using_dbt_utils_macros dbt model

Run Tests in dbt

Another benefit of using dbt is the ability to test your data. Out of the box, dbt have the following generic tests: unique, not_null, accepted_values and relationships. An example of these tests on the model is shown below:

version: 2

models:
    - name: covid19_latest_stats
      description: "A model of latest stats for covid19"
      columns:
          - name: iso_code
            description: "The country code"
            tests:
                - unique
                - not_null
schema.yml (dbt testing)

You can run the tests via dbt test. You can see the output below

Results of running dbt test on the dbt Cloud dashboard
Results of running dbt test on the dbt Cloud dashboard

For more information on testing in dbt, you can visit dbt documentation.

Generate Documentation in dbt

You can generate documentation for your dbt project by simply running dbt docs generate in the command section as shown below:

Generate documentation for a dbt project
Generate documentation for a dbt project

You can browse through the generated documentation by clicking on view docs. You can see an overview of the generated docs below.

The documentation generated by the command dbt docs generate.

In addition to dbt docs generate, dbt docs can also serve a webserver with the generated documentation. To do so, you need to simply run dbt docs serve. More information about generating docs for your dbt project is available here.

Other Features

Database administration using Hooks & Operations

There are database management tasks that require running additional SQL queries, such as:

  • Create user-defined functions
  • Grant privileges on a table
  • and many more

dbt has two interfaces (hooks and operations) for executing these tasks and importantly version control them. Hooks and operations are briefly introduced here. For more info, you can check dbt documentation.

Hooks

Hooks are simply SQL snippets that are executed at different times. Hooks are defined in the dbt_project.yml file. Different hooks are:

  • pre-hook: executed before a model is built
  • post-hook: executed after a model is built
  • on-run-start: executed at the start of dbt run
  • on-run-end: executed at the end of dbt run

Operations

Operations are a convenient way to invoke a macro without running a model. Operations are triggered using dbt run-operation command.

Note that, unlike hooks, you need to explicitly execute the SQL in a dbt operation.

Conclusion

dbt is a nice tool that is definitely worth giving a try as it may simplify your data ELT(or ETL) pipeline. In this post, we learned how to set up and use dbt for data transformation. I walked you through the different features of this tool. In particular, I provided a step-by-step guide on

  • Configuring a dbt project
  • Creating dbt models (SELECT statements)
  • Build complex dbt models using global variables and macros
  • Building complex models by referring to other dbt models
  • Running tests
  • Generating documentation

You can find the GitHub repo containing all scripts (including the data ingestion script) below. Feel free to fork the source code of this article.

👉

This article is also published on KDnuggets.

Thanks for reading 🙏

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