ER Or Entity-Relationship Diagram in SQL

Definition: An entity-relationship (ER) diagram is a specialized graphic that illustrates the interrelationships between entities in a database. ER diagrams often use symbols to represent three different types of information. Boxes are commonly used to represent entities. Diamonds are normally used to represent relationships and ovals are used to represent attributes.

Also Known As: ER Diagram, E-R Diagram, entity-relationship model

Examples:

Consider the example of a database that contains information on the residents of a city. The ER digram shown in the image above contains two entities -- people and cities. There is a single "Lives In" relationship. In our example, due to space constraints, there is only one attribute associated with each entity. People have names and cities have populations. In a real-world example, each one of these would likely have many different attributes.