Most complaints I hear regarding SQL can usually be classified in one of the following categories:

• The complainer lacks a full understanding or appreciation of set and relational theory
• The complaint is made in relation to a weak SQL implementation

That may or may not be true but Date is one of the authors of the SQL standard and one of the leading authors of RDBMS books, so I think we can assume that his criticisms are not those of a "crank".

One thing that does annoy me about Darwen and Date's criticisms is that they are a bit too fond of making statements about facts they have already established and just refering the reader to the paper/article/expensive-book-that-they-just-happen-to-have-written in which they were established. They are struggling against a huge mainstream of standard SQL/RDBMS dogma they need to make their arguments easily accessible and not put the burden on the reader - that's fine for academia with it's well-stocked libraries but a bit much to expect of the casual reader who has not yet drunk their kool-aid.