题目内容
The data models are divided into three classes：____________ ____________ models, ____________ ____________ models and ____________ ____________ models.
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An ____________ is an object, which can be uniquely distinguished from other objects.
The set of all entities of the same type and relationships of the same type are termed as ____________ ____________ and ____________ ____________ respectively.
Entities and relationships are to be distinguished using the concept of ____________ ____________.
Object-based logical models are used for describing data at the ____________ and ____________ levels.
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