ehrQL tutorial🔗

Danger

This page discusses the new OpenSAFELY Data Builder for accessing OpenSAFELY data sources.

Use OpenSAFELY cohort-extractor, unless you are specifically involved in the development or testing of Data Builder.

OpenSAFELY ehrQL and its documentation are still undergoing extensive development. We will announce when ehrQL is ready for general use on the Platform News page.

Table of contents🔗

Todo

Remove this.

We should incorporate this as a full part of the documentation, so that it is easy to discover and follow on to the next page.

This will be when this documentation is ready (or close to ready) for public use.

Audience🔗

This tutorial is aimed at:

new users of ehrQL who wish to write dataset definitions to extract data from OpenSAFELY backends
developers involved in operating data backends who wish to understand more about how OpenSAFELY backends are accessed by users

There is no requirement for approval or access to OpenSAFELY to follow this tutorial. This tutorial uses artificial data throughout.

This tutorial assumes that you have some familiarity with programming or scripted data analysis, whether that is in Python, R, Stata or any other language.

Familiarity with:

OpenSAFELY
How electronic healthcare records are structured

would also be helpful, but is not essential.

How to work through this tutorial series🔗

The tutorial is split into 7 sections that we recommend you work through sequentially. Each section builds on previous knowledge. Throughout the tutorial series, you will see optional explanations and reading, that you might want to explore at the time or afterwards.

We recommend that you work through the tutorial by installing ehrQL, downloading the sample code, and running the code on your machine.

Learning objectives🔗

This tutorial series will introduce you to writing and running ehrQL queries with ehrQL.

By the end of the tutorials, you will be able:

to install ehrQL
to write a dataset definition in ehrQL
to run a dataset definition with ehrQL
to use electronic health record codelists in ehrQL
to perform queries via selecting, filtering and aggregation

Each tutorial section will have its own learning objectives.

Installation🔗

To try running or modifying the examples in this tutorial, you will need to install ehrQL and have a text editor.

Installing ehrQL🔗

ehrQL runs on Windows, macOS, and Linux either via:

the OpenSAFELY CLI (recommended)
a manually installed Python package

OpenSAFELY CLI🔗

The OpenSAFELY CLI requires a working Docker installation. If you do not already have the OpenSAFELY CLI installed, refer to the instructions. Once you have the OpenSAFELY CLI installed, you are ready to use ehrQL.

Install via Python🔗

If you are unable to install Docker, you can try ehrQL via Python.

A Python package install will still allow you to follow this tutorial, but will not allow you to run full OpenSAFELY projects.

We will not cover the Python installation here. Refer to the separate page on this.

Text editor🔗

You can edit dataset definitions in any text editor.

You may find it useful to use a text editor or integrated development environment that supports Python syntax.

If you have no preference already, Visual Studio Code is a reasonable, free-of-charge choice that runs on Windows, macOS and Linux.

How to download the tutorial code and data🔗

Throughout this tutorial, we will refer to code examples.

These examples are available for you to run and to experiment with.

When working through each tutorial, you may find it useful to open the relevant dataset definition and example CSV data in a text editor, to refer along with.

Download the documentation source code that contains these examples.
Open the .zip file.
Extract the ehrql-tutorial-examples somewhere.
In the ehrql-tutorial-examples directory that you extracted, delete the outputs directory. This contains outputs that have been generated already for the dataset definitions. We will regenerate these throughout this tutorial.
Navigate to the ehrql-tutorial-examples directory that you extracted when working in a terminal.

Each dataset definition used in this tutorial has a filename of the form:

IDENTIFIER_DATASOURCENAME_dataset_definition.py

For example, for

1a_minimal_dataset_definition.py

the identifier is 1a and the data source name is minimal. The identifier associates the dataset definition with a specific tutorial page.

How to run the tutorial code🔗

There are two ways to run the tutorial code:

Run the code directly using opensafely exec
Run the code via project.yaml file (this is how real OpenSAFELY projects get run)

Run using `opensafely exec`🔗

To run this dataset definition with ehrQL,

In a terminal, enter the ehrql-tutorial-examples directory that you extracted from the sample data.
Run this command:

opensafely exec ehrql:v0 generate-dataset "1a_minimal_dataset_definition.py" --dummy-tables "example-data/minimal/" --output "outputs.csv"

3. You should see ehrQL run without error and find the outputs.csv file in the ehrql-tutorial-examples directory that you were working in.

Tip

In general, the command to run a dataset defintion looks like:

opensafely exec ehrql:v0 generate-dataset "IDENTIFIER_DATASOURCENAME_dataset_definition.py" --dummy-tables "example-data/DATASOURCENAME/" --output "outputs.csv"

You need to substitute DATASOURCENAME with the appropriate dataset name, and IDENTIFIER_DATASOURCENAME_dataset_definition.py to match the specific dataset definition that you want to run.

Tip

The output in this example is called outputs.csv, but you can choose any valid filename.

Run via `project.yaml`🔗

Each OpenSAFELY project has a project.yaml (see projects), where actions are defined. The project.yaml file is run via the opensafely run command.

For this tutorial, we have already defined individual actions in the project.yaml for each example, for convenience.

To run this dataset definition with ehrQL:

In a terminal, enter the ehrql-tutorial-examples directory that you extracted from the sample data.

Run this command:

opensafely run extract_1a_minimal_population

The OpenSAFELY CLI now runs and should complete without error. This command runs the appropriate action in the project.yaml file.
You should be able to find the relevant output file in the ehrql-tutorial-examples directory that you were working in. The filename of the output is also specified in the project.yaml.

Tip