Data modelling schema
A data modelling schema is a method that allows us to model or illustrate a database. This is often in the form of a graphic diagram, but other means of communication are also desirable; people who are not in the computer field may or may not understand diagrams and graphics. The ER diagram is a graphic tool that facilitates data modelling. ER diagrams are a subset of “semantic models” in database parlance. Semantic models refer to models that intend to elicit meaning from data. ER diagrams are not the only semantic modelling tools but are common and popular.
When we begin to discuss the contents of a database, the data model helps us to decide which piece of data goes with which another piece (s) of data on a conceptual level. An early concept concerning the database is to recognize that there are levels of abstraction we can use in discussing databases.
For example, if we were to discuss the filing of “names,” we could discuss (a) abstractly, that is, “We will file names of people we know”; or (b) concretely, that is, “We will file first, middle, and last names (20 characters each) of people we know, so that we can retrieve the names in alphabetical order by last name, and we will put this data in a spreadsheet format in package x.” If a person is designing a database, the first step is to abstract, then refine the abstraction.
The longer one stays away from the concrete details of logical models (relational, hierarchical, network) and physical realizations (fields [how many characters, the data type…. and files (relative, spread-sheet, … the easier it is to change the model and to decide how the data will eventually be physically realized (stored). When we use the term field or file, we will be referring to physical data as opposed to conceptual data.
Mapping is the process of choosing a logical model and then moving to a physical database file system from a conceptual model (the ER diagram). A physical file loaded with data is necessary to actually obtain data from a database. Mapping is the bridge between the design concept and physical reality. In this book, we concentrate on the relational database model due to its ubiquitousness in the contemporary database models.
So, What Is an Entity Relationship Diagram?
The ER diagram is a semantic data modelling tool used to accomplish the goal of abstractly describing or portraying data. Abstractly described data is called a conceptual model. Our conceptual model will lead us to a “schema.” A schema implies a permanent. fixed description of the structure of the data. Therefore, when we agree that we have captured the correct depiction of reality within our conceptual model, our ER diagram, we can call it a schema.
An ER diagram could also be used to document an existing database by reverse engineering it, but in introducing the subject, we focus on the idea of using an ER diagram to model a to-be-created database, and we deal with reverse engineering in further discussion.