Semi-structured Data Model
The semi-structured data model is designed as an evolution of the relational data model that allows the representation of data with a flexible structure. Some items may have missing attributes, others may have extra attributes, some items may have two or more occurrences of the same attribute. The type of an attribute is also flexible: it may be an atomic value, or it may be another record or collection. Moreover, collections may be heterogeneous, i.e., they may contain items with different structures. The semi-structured data model is self-describing data model, in which the data values and the schema components co-exist. Formally:
A semi-structured data instance is a rooted, directed graph in which the edges carry labels representing schema components, and leaf nodes (i.e., nodes without any outgoing edges) are labeled with data values (integers, reals, strings, etc.).
There are two variations of semi-structured data, depending on...
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