You can set environment variables for a project by adding a special type of Integration. Go to Integrations in the left sidebar, and then either modify an existing "Environment variables" integration or add a new one:
Don't forget to connect to the newly created "Environment variable" integration.
Environment variables are a secure way to store secrets. They are stored encrypted, and decrypted only before mounting them into your project.
You might be storing your data in a database which does not yet have a native integration in Deepnote. Many times, you can:
Find a python package which provides connectivity to the database or data source.
Store the secrets as environment variables.
When you mount the environment variables to the project, you can use them to authenticate access to your database provided by the native python package.
A similar approach was follow in the Deepnote's Snowflake integration.