An ER diagram is a means of visualizing what kind of information a system produces and how that is related.


Those are the main components of an ERD:


    • Entities, which are represented by rectangles. An entity is an object or concept about which you want to store information and it translates to a table in a relational database.
    • Attributes, which are drawn inside entities and represents characteristics of the entity. A key attribute is the unique, distinguishing characteristic of the entity.
    • Relations, which are represented by connecting lines between entities, show how two entities share information and relates to one another in the database. It is possible to have self-linked entities, for hierarchical structures like employee or bill of materials (BOM).
    • Cardinality, specifies how many instances of an entity relate to one instance of another entity. This often includes ordinality, while cardinality specifies the occurrences of a relationship, ordinality describes the relationship as either mandatory or optional. In other words, cardinality specifies the maximum number of relationships and ordinality specifies the absolute minimum number of relationships.



There are many notation styles used to represent relation cardinality, Zamolxis loosely applies the Information Engineering notation:


Symbol

Description

zero or one

only one

many

one or more