Re: database design method

Paul Vernon wrote:
>"Jan Hidders" <hidders_at_REMOVE.THIS.uia.ua.ac.be> wrote in message
>news:3dcff3de$1_at_news.uia.ac.be...
>> If you want to be precise it should be for example:
>>
>> ( node-value : "book",
>> subtrees : { ( node-value : "chapter1", subtrees : {} ),
>> ( node-value : "chapter2", subtrees : {} ) } )
>>
>> and in this value the following nested values would also belong to the type:
>>
>> ( node-value : "chapter1", subtrees: {} )
>>
>> ( node-value : "chapter2", subtrees: {} )
>>
>> Note that his example is a bit artificial because the chapters are more
>> likely to be list then a set, but that is besides the point.
>
> So {} is a valid value for this tuple type. Excuse my ignorance, but what is
>it? A tuple with two attributes but no 'rows'?!

Look in the type definition. The type of 'subtrees' is a RELATION type. The {} denotes the empty set, i.e., an empty relation.

• Jan Hidders