Start with relational, understand why it is the reference architecture, and from there the tradeoffs involved and what other architectures bring to the table (columnar, streaming, object, in-memory, array, distributed, blockchain, nosql, etc)
To really understand why you should start with relational, read Stonebraker's classic paper: "What goes around comes around": https://people.cs.umass.edu/~yanlei/courses/CS691LL-f06/pape...
It will teach you database evolution history so that you don't end up reinventing the wheel.
Stonebraker's MIT course: https://ocw.mit.edu/courses/electrical-engineering-and-compu...
There are a few lectures of this course in youtube, not by him: https://youtube.com/playlist?list=PLfciLKR3SgqOxCy1TIXXyfTqK...
MIT's distributed systems course also touches on databases: https://pdos.csail.mit.edu/6.824/schedule.html
Course by one of his disciples: https://15721.courses.cs.cmu.edu/spring2020/
Yet another disciple (in edx too): https://m.youtube.com/playlist?list=PLYp4IGUhNFmw8USiYMJvCUj...
The red book: http://www.redbook.io/
For learning SQL really well, including relational algebra, I like this course: http://users.cms.caltech.edu/~donnie/cs121/