Definition
Entity Relationship Model
An entity-relationship model (or ER model) describes interrelated things of interest in a specific domain of knowledge.
Entity Set
Entity Set
An entity set is a concrete set of entities of the same entity type.
All entities of the same entity type (or entity set) have the properties.
Entity
Entity
An entities is an object of the real world for which information is stored. Only properties of entities can be stored in a database, not the entity itself.
Weak Entity
Attribute
Attribute
Single-Valued Attribute
Single-Valued Attribute
An attribute is called single-valued if it accepts only one concrete value.
Multi-Valued Attribute
Single-Valued Attribute
An attribute is called multi-valued if it accepts a collection of values.
Composite Attribute
Composite Attribute
An attribute is called composite if is consists of multiple sub-attributes.
Example:
Derived Attribute
Derived Attribute
Key
Primary Key
Primary Key
A primary key is a key candidate that identifies an entity uniquely.
Relation
Weak Relation
IS-A
Total IS-A
Total IS-A
Every less-specialised entity type must be owned by a specialised entity type.
Partial IS-A
Partial IS-A
Every less specialised entity type can (but doesn’t have to) be owned by a specialised entity type.