Suppose you had an event object that had the following properties: class event: def __init__(self, what, where, when) self.what=what self.where=where self.when=when Now, in the above imagine that 'when' is a date and 'where' is a URI. But suppose in your domain the 'what' could be one of three types: a URI: this means something well known enough that we have a pre-canned name for it. a String: this means it's some kind of opaque literal, like a java stacktrace or hexed coredump, or maybe some chunk of XHTML. some XML format: this means the event has a 'well-known' XML format that we know how to process, ie some SOAP thing or an Atom entry. The interesting thing here is that these 3 types are disjoint; they're not specializations of some common form, modulo the crashingly trivial observation they could all be represented as text. In code or markup you'd probably have a some extra metadata about the 'what' that would allow you to switch-on-type For example, in XML that could result in different element names altogether for each type maybe or some @type attribute, or in RDF, it would an rdf:type annotation. So given that the range of value types can be disjoint I was wondering what people think the idiomatic way to represent this event structure in an RDBMS would be. Have a foreign-key for the 'what' column pointing to 1 of 3 possible tables? Make the 'what' column a blob? Change the design not to have use disjoint types? Have the three what tables hold foreign keys back the event table? Or some other approach?...
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