DBMS
Entities, Entity Sets and Entity Types
An entity is a distinguishable thing the database stores facts about, an entity type is its definition, and an entity set is the collection of entitie...
ER Model
Entities, attributes, relationships, cardinality, participation, weak entities, ER diagrams and the extended ER model.
9 notes
Notes
DBMS
Types of Attributes in the ER Model
Attributes are simple or composite, single valued or multivalued, stored or derived, and may be complex, key or null valued. Each type converts into t...
DBMS
Keys in the ER Model
A key attribute distinguishes one entity from another. Understanding super keys, candidate keys, primary keys and partial keys at design time prevents...
DBMS
Relationships, Relationship Sets and Degree
A relationship is an association among entities. Relationship types have a degree, may carry their own attributes, and may connect an entity type to i...
DBMS
Cardinality and Participation Constraints
Cardinality says how many entities on one side may relate to entities on the other; participation says whether every entity must take part. Together t...
DBMS
Strong and Weak Entities
A weak entity has no key of its own and can only be identified through an owner entity. It is drawn with a double rectangle, joined by an identifying...
DBMS
Drawing and Reading ER Diagrams
A complete guide to ER notation and a repeatable eight step method for turning a written requirement into a correct, readable ER diagram.
DBMS
Extended ER: Generalisation, Specialisation and Aggregation
The extended ER model adds inheritance to ER. Specialisation splits an entity into subclasses, generalisation combines subclasses into a superclass, a...
DBMS
Converting an ER Diagram into Relational Tables
Seven rules turn any ER diagram into a relational schema: entities become tables, one to many puts a foreign key on the many side, many to many become...