I don't think Slick can be considered a very SQL-centric API. While the Slick folks embrace the relational model, they certainly do not embrace the SQL language. Just as with LINQ, this can be desirable if you want to reason about collections in a more general sense. On the other hand, the SQL standard has gone far beyond the "occasional" OUTER JOIN that can turn out to be a true challenge to Slick.

In other words, SQL has never been purely relational. It is a beast of its own.