2. Entity
The word entity in the ERM 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.
3. Attributes
Required attributes- an attribute that must have a value, ,
it cannot be left empty.
STUDENT
STU_LNAME
STU_FNAME
STU_INITIAL
STU_EMAIL
STU_PHONE
4. Optional attributes- attribute that does not require a
value, it can be left empty.
STUDENT
STU_LNAME
STU_FNAME
STU_INITIAL
STU_EMAIL
STU_PHONE
5. Domains- is the set of possible values for a given attribute.
Identifiers (Primary key)- one or more attributes that
uniquely identify each entity instance.
Composite identifiers- a primary key composed of more
than one attribute.
6. Composite attributes- can be further subdivided to yield
additional attributes.
For example the attribute ADDRESS can be subdivided into
street, city, state, zip code.
Simple attributes- cannot be subdivided.
For example, age, sex, marital status.
7. Single-valued attributes- can have only a single value.
For example, a person can only have one SSS Number.
Multivalued attributes- can have many values.
For instance, a person may have a several college degrees,
and a household may have different phones, each with its
own number.
8. CHEN MODEL CROW’S FOOT MODEL
MOD_CODE CAR_YEAR CAR
PK CAR_VIN
CAR_VIN CAR CAR_COLOR
MOD_CODE
CAR_YEAR
CAR_COLOR
9. Derived attributes – whose value is calculated (derived)
from other attributes.
It is derived by using an algorithm.
Sometimes referred to as computed attributes.
10. Relationships
The entities that participate in a relationship are also
known as participants, and each relationship is identified
by a name that describes the relationship.
11. Relationship name is an active or a
passive verb.
Relationship name:
writes
Author Book
An author writes one or more books
A book can be written by one or more authors.
12. Connectivity and Cardinality
Connectivity used to describe the relationship
classification.
Cardinality expresses the minimum and maximum number
of entity occurrences associated with one occurrence of
related entity.
The first value represents the minimum no. of associated
entities. The second value represents the maximum no. of
associated entities
