I guess the place I'd start is with what CJ Date has been saying about "Tutorial D" and how much you can rephrase into D.
In someways it's a very orthogonal concept, we tend to think of databases as "places where we store data".
What Date is saying is, no, they are collections of predicates and an algebra for doing quantified logic operations on the them.
The critical idea behind "algebra" is closure. You operate on relations (tables in SQL speak) and the result is a relation which you can use in further operations.
In D, that concept is almost trivial.
It has taken SQL ages to come up with half-assed things like views and temp tables and sub-selects.
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