Database Dr. Roueida Mohammed

The Entity Relationship Model The entity relationship model (or ER model) is a way of graphically representing the logical relationships of entities (or objects) in order to create a database.

Elements of the E/R Model Entity sets. Attributes. Relationships

Entity-Relationship Diagrams An E/R diagram is a graph representing entity sets, attributes, and relationships. Elements of each of these kinds are represented by nodes of the graph, and we use a special shape of node to indicate the kind, as follows: Entity sets are represented by rectangles. Attributes are represented by ovals. Relationships are represented by diamonds

RELATIONSHIP SETS one-to-one one-to-many many-to-one Many-to-many

Design Principles Faithfulness Avoiding Redundancy Simplicity Counts Choosing the Right Relationships Picking the Right Kind of Element

The Modeling of Constraints Keys Single-value constraints Referential integrity constraints Domain constraints General constraints

Weak Entities