Many of our projects use relational databases.
The PostgreSQL Tutorial provides a nice introduction to SQL and to Postgres. Sections 3.5 and 3.6 are less relevant.
The Django Documentation about Models provides a nice description of how the Django ORM relates to the underlying database.
This Stanford Course is a great place to learn databases in-depth, including the mathematical foundations of relational databases. There is too much information here for practical day-to-day usage, but if you have time time and inclination, this course lays a great foundation. Articles that provide a conceptual basis for how CSS works:
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