What is an ERD?
An Entity Relationship Diagram, or ERD is a schematic model of the data requirements for a database. An ERD does not focus on implementation.
ERD Objects:
Entity, such as product, employee, customer, or film. In SQL an entity is a TABLE.
Relationship describes the “link” between entities, for example customer buys product. In SQL a relation is a foreign key.
Attribute is an attribute of the entity, such as product ID. In SQL an attribute, or field, is a COLUMN of the TABLE.
In an ERD entities are rectangles, relationships are lines between the rectangle, and attributes are added inside the rectangles.
An ERD is usually accompanied by a Data Dictionary, which defines the names and descriptions of ERD objects in text format.
An instance is a specific statement about specific entities, as for example. “Student Janelle Adams registers for CS 100 Section 02 with Instructor Ana Davis.” Therefore, an entity can have multiple instances.