The key attributes have unique values for
all entities of the same type.
The key attributes identify the entity.
Entities without key attributes are weak entities. Often weak entities are things associated with
other things, such as extras in a movie.
Entities are drawn as rectangles. The weak ones are bordered by doubled
Relations connect entities. They are drawn with diamonds.
Identifying relationships, drawn
with two sides, connect weak entities to the
source of their identity.
Attributes are drawn with ovals connected to entities.
Multivalued attributea are drawn inside double ovals.
The arcs connected to the relationships are also labeled with
cardinalities similar to those seen in UML diagrams.
Sometimes “crows feet” are used to denote a
one-to-many relation. These are often used with
ER Design tools
Many database management systems have data modeling tools for
documenting databases and drawing ER diagrams.
workbench can do this.
These models can be used to
generate SQL statements to create the database.