Ted Codd's big idea is fairly simple: "we can store data in logical, rather than physical, structures and then use logic statements, rather than algorithms, to retrieve the data."
Wikipedia can often end being more confusing than enlightening but the "informal introduction" on this page is a place to start:
Here's a page I just found about predicate/first-order logic:
This book by Chris Date covers a lot of basic relational database ideas, but his writing is a bit tedious:
After that the main mental challenge is just learning to think in sets. Joe Celko has some writings specifically about that in his various books.
After that, the best book I've seen for SQL and practical issues is The Art of SQL:
Hope that helps. There's a lot of really bad writing about relational databases out there.
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