2.1 RDF Data Model
RDF(Powers, 2003) is a framework for metadata that provides interoperability between applications by machine-understandable web data (Kim et al., 2017). The RDF definition can be expressed as:
(Subject, Predicate, Object)∈ (U ∪ B) × (U ∪ B ∪ L)
(1)Where U represents the collection of URI, B is a collection of anonymous resources, and L is a collection of literal property values.
In addition to the storage of triples, scholars have proposed to store RDF data as property tables which are shown in Table 1. In the table 1, the first column is the subject in the RDF triple, the remaining columns are the properties of the subject, and each row corresponds to the subject and its corresponding property value in turn (Tathiane et al., 2018). The storage method of the property table can effectively reduce the number of connections and improve the query speed when processing the query requests.
Table 1. RDF property table storage method
subject | predicate1 | predicate2 | predicate3 | predicate5 | predicate6 | predicate7 | … |
subjec1 | Object1 | | Object2 | | | Object7 | |
subjec2 | Object2 | Object2 | Object4 | | | | |
subjec3 | Object2 | | | Object6 | Object9 | Object10 | |
… | | | | | | | |