Re: What does "optional" mean?
On 26/02/2012 14:11, Costello, Roger L. wrote: > Hi Folks, > > Every schema language (DTD, XSD, RNG) has the notion of optional -- optional elements and optional attributes. > > But what does it mean to declare an element or attribute optional? > > Go and read the relational database literature from the early 80s (?) when there was a religious war about the meaning of null, with Ted Codd and co writing papers advocating that there should be four or five different kinds of null with different meanings. Then when you've read them, chuck them in the bin. Just as the attribute iubce='qwudob' means whatever the sender and recipient agree it should mean, so equally the absence of the iubce attribute means whatever the sender and recipient agree it to mean, which might be "I don't know", or "not applicable", or "I don't think you want to know this", or "you already know this so I'm not telling you again", or perhaps it means exactly the same as iubce='qwudob'. Michael Kay Saxonica
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