What it was
A group project for the database systems course in my degree. We modelled the things a university library has to track, drew the extended entity-relationship diagram in MySQL Workbench, turned it into a relational schema, and wrote the SQL that creates the tables, loads sample data and answers the everyday questions a librarian asks of the system. The repo holds the Workbench models, the diagrams, the SQL scripts and the written report we handed in.
What I learned
This was where SQL stopped being syntax and became modelling. Deciding what a row means, where a foreign key belongs and which constraints protect the data was the real work; the queries followed from a good schema. I still start every warehouse or lakehouse design with the same questions about grain and keys, only now with many more tables and with the diagram living in dbt rather than in Workbench.