Database Modeling: Conceptual, Logical, and Physical Levels
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Database Modeling Levels
- Conceptual Level: Focused on describing the real world regardless of hardware and software. It is an analysis model represented by the Entity-Relationship model.
- Logical Level: Aims to describe data for a particular database model (Relational, Hierarchical, CODASYL). The most widely used is the Relational Model.
- Physical Level: Hardware-oriented and specific to a particular DBMS (e.g., Access, Oracle, Informix).
Entities
An Entity is any object, real or abstract, that exists in reality and about which we have information (e.g., employees, students).
- Regular Entities: Exist independently.
- Weak Entities: Their existence depends on another type of entity. For example, an employee's family depends on the employee; if the employee is removed from the database, their dependents are removed as well.
Each particular embodiment of an entity is called an Occurrence or Instance.
Relationships
A Relationship is the association, connection, or correspondence between entities. There may be multiple relationships between entities, and an entity may be related to itself.
Relationship Properties
- Degree of Relationship: The maximum number of entities involved in a relationship (e.g., degree 1, 2, 3).
- Role: The function that each entity performs in the relationship.
- Type of Correspondence: The maximum number of occurrences of each entity involved in a relationship.
Cardinality Types
- 1:1 (One-to-One): A man is married to a woman and a woman to a man.
- 1:N (One-to-Many): An employee belongs to a department, but a department can have several employees.
- N:M (Many-to-Many): An employee may work on many projects, and a project can have many employees.
Note: Many current DBMS do not directly implement N:M relationships (as they often resolve into a new entity with 1:N relationships) or even 1:1 (which is treated as a specific case of 1:N). When more than one occurrence is involved, the bond between the entity and the relationship is oriented.