The consensus among relational theorists appears to be that data types can be arbitrarily complex. For instance, C.J Date and Hugh Darwen write the following ([1] page 56):
Third, we remind you that types are not limited to simple things like integers. Indeed, we saw in Chapter 1 that values and variables can be arbitrarily complex—and that is so precisely because the types of those values and variables can be arbitrarily complex. Thus, to paraphrase a remark from that chapter, a type might consist of geometric points, or polygons, or X rays, or XML documents, or fingerprints, or arrays, or stacks, or lists, or relations (and on and on).
