How can properties that have multiple values be expressed?
A resource may have multiple values for a specific property. E.g. a set of keywords associated with a research paper. RDF offers several options for modelling these multi-valued relations. Sometimes these multi-valued relations have an explicit ordering that should be preserved in the data, e.g. the order of authors. In other cases an explicit ordering is not required
For example a research paper may have multiple keywords. The ordering of these keywords is not important, so using the Dublin Core subject property, we can express this multi-valued relation as:
_:doc dc:subject "RDF"; dc:subject "Semantic Web".
Repeating properties is the simplest approach to handling multi-valued relations. The alternatives all have their downsides. One alternative would be to use a structured value for the literal, e.g:
_:doc dc:subject "RDF, Semantic Web".
But structured values limit the ability for applications to query for specific data items, e.g. documents that have a specific keyword. With a structured value a regular expression would be required to test for the values in the literal.
Another alternative would have been to use an RDF Container (Sequence, Bag, or Alt) or Collection (RDF List). Some of these structures imply ordering (Sequence, List) while others (Bag, Alt) don't imply an ordering over their members. This means that the keywords could have been expressed as:
_:doc dc:subject ( "RDF", "Semantic Web" )
There are several downsides to using Containers and Collections. Firstly they can be difficult to use as they can be hard to query. Secondly, when combining data from different sources the Containers and Collections won't be merged together to create a single structure; the lists will remain separate.
On the whole, the simplest way to express multi-valued relations is to use a simple Repeated Property. Collections and Containers are best used only when ordering is significant.