ENTITY – RELATIONSHIP MODEL Melissa Ann D. Callejo Chairmane Abanilla

 Is a specialized graphic that illustrates the relationships between entities in a database.  often use symbols to represent three different types of information. BOX - commonly used to represent entities Diamond - used to represent relationships

OVAL - used to represent attributes

 ENTITY - is an existing or real thing - used to identify units, whether concrete things or abstract ideas, that have no ready name or label

In Entity Relationship Model Entity : > correspond to a table - not a row- in a relational environment. The ERM refers to a table row as an entity instance or entity occurrence.

ENTITY INSTANCE Example ENTITY – PEOPLE INSTANCE – JOHNNY DEPP

Entity Occurrence  is an instance of an entity Billy Jones (ie: SN12345, Billy, Jones, 18/08/1950) is an entity occurrence. An entity occurrence can also be referred to as a record.

 ATTRIBUTES -information about each entity

 If the font is boldface, these are required fields.

 If the attribute is underlined, it indicates that it serves as the database’s primary key.

PRIMARY KEY

Optional Attributes  attribute that does not require a value, it can be left empty.  For example: PhoneNumber

Composite Attributes  Can be subdivided to yield additional attributes.  Example:  Address can be divided into Street, city, state, zipcode

Simple Attributes  Cannot be subdivided  Example: age, gender, Religion

Single-Valued Attributes  Can have only a single value Example: Student and ID Number

Multivalued Attributes  Attribute that hold multiple values.  A customer can have multiple phone numbers, id's etc

Stored and Derived Attributes  Stored Attribute: An attribute that supplies a value to the related attribute. > Date of Birth  Derived Attribute: An attribute that’s value is derived from a stored attribute. > age, and it’s value is derived from the stored attribute Date of Birth.

Complex Attribute  A complex attribute that is both composite and multi valued.

Chen and Crow’s Foot Notation Chen’s Notation

Crow’s Foot Notation

references  products/l/blentity-relationship- diagrams.htm products/l/blentity-relationship- diagrams.htm  types.html