Relational model too, yes. But the biggest "aha" moments I've had mentoring devs on thinking in SQL came when they understood that the result of a query is itself a set. So you can join two selects, join that result with another select, then group on the whole thing.

Those kinds of patterns would often replace manual loops in stored procedures and opened up set theory-focused way of thinking. These patterns are made more performant and easier with good relational modeling, of course.