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Entities
Oracle Tips by Burleson Consulting |
Entities
Entities
are objects or classes of objects
that can be identified. They can be real things (an employee) or
logical things (a department). An employee is a class of entities.
Bill, Tom and Sue are employee entities. The entity class becomes the
tables while the entities are the rows. An entity class has
attributes or elements that make up the entity.
The attributes become the columns in the tables.
An employee has a name, ssn,
address, hire date, etc. It also needs to be uniquely
identified so we can tell the difference between Bill and Sue, so we
will give each employee a unique employee number. Figure 1.8 is the
employee entity.
In the above example, the employee entity has the
attributes defined as emp_num
(employee number), ssn,
emp_name,
hire_date and
dept. The primary key
(unique identifier) is the employee number, identified as the key
because it is underlined. There are some key things to note about the
employee entity.
First, all employees are going to be in a
department, so the dept
attribute will have a lot of redundant information in it. Also, the
department is not dependent on the
emp_num key, so we need to split it out into its own entity.
Secondly, you might want to use the social security number (ssn)
as the primary key
; however, this will limit your
design to just persons with a ssn.
No persons from other countries, since they will not have a
ssn. Finally, employee name
should be broken up into subelements such as first, middle initial and
last name. After incorporating the changes, we now have the diagram
in Figure 1.9.
Notice that department is now its own entity with
the dept_num as the primary
key. Also notice that we place a
dep_num attribute in the
employee entity, this is know as a Foreign Key. It
is an attribute that references another entity’s primary key (more on
that in Chapter 5). This actually defines a relationship between the
employee and the department entity.
But, I am ahead of myself. Entities can be a
type of another entity. An employee is also a person. If I have a
person entity, the employee would be a subtype of the person entity.
For now, we will stick with the employee type.
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