A semi-structured data model is based on an organization of data in labeled trees (possibly graphs) and on query languages for accessing and updating data. The labels capture the structural information. Since these models are considered in the context of data exchange, they typically propose some form of data serialization, i.e., a standard representation of data in files. Indeed, the most successful such model, namely XML (that is promoted by the W3C), is often confused with its serialization syntax. XML equipped with query/update language  is a semi-structured data model.
Semi-structured data models are meant to represent from very structured to very unstructured information, and in particular, irregular data. In a structured data model such as the relational model , one distinguishes between the type of the data (schema in relational terminology) and the data itself (instance in relational terminology). In semi-structured data models, this...
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