All database need models and schemas. Models and schemas are what provides the context like relationships and details of the data items and is one of database system elements. The models are stored in the catalog of database management systems. A database model will have multiple data models. Data model example will be employee, which have characteristics like age, date hired, position number and duty title.
There are three models one should be aware of, which are conceptual data model, logical data model, and external data model. Each describes data from different perspective and ensures clear description of the data items and its relationships with other data items. Conceptual data model is a big pictures description that is usually represented using enhanced entity relationship model or object oriented model. Logical data model is database management system specific model that maps conceptual data model towards specific implementation environment. Logical can be mapped to internal data model that presents data’s physical storage details. External is simply subsets of data items in logical model.
Another element is three layer architecture that is broken down into internal, logical, and external layers. Internal layer focuses on how data are stored physically. Logical layer focuses on data items and details. External layer focuses on view, that filters outer data items to ensure right data goes to the right people and ensures both security and simplicity. View part of external layer may be connected to one or more data items within logical layer, but all data items in logical layer are connected to internal layer. Catalog is what stores all of these items and synchronizes them for consistency.
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