Page 100 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Unit 6: Relational Language and Database Design

the participation of the entity Department in the relationship manages is said to be total. If the Notes
participation is not total, then the participation is said to be partial. A partial participation
example is the participation of the entity set Employees in the relationship Manages, thus every
employee cannot be a manager of a department.

6.8 E-R Diagrams

Now we are in a position to write the ER diagram for the Company database which was introduced
in the beginning of this unit. The readers are strictly advised to follow the steps shown in this
unit to design an ER diagram for any chosen problem.
Step 1: Identify the Strong and Weak Entity Sets

After careful analysis of the problem we come to a conclusion that there are four possible entity
sets as shown below:
1. Employees Strong Entity Set

2. Departments Strong Entity Set
3. Projects Strong Entity Set
4. Dependents Weak Entity Set

Step 2: Identify the Relevant Attributes

The next step is to get all the attributes that are most applicable for each entity set. Do this work
by considering each entity set in mind and also the type of attributes. Next job is to pick the
primary key for strong entity sets and partial key for weak entity sets.

Example: Following are the attributes:
1. Employees SSN. Name, Addr, DateOfBirth, Sex, Salary
2. Departments DNo. DName, DLocation
3. Projects PNo. PName, PLocation
4. Dependents (weak) DepName, DateOf Birth, Sex, Relationship
The underlined attributes are the primary keys and DepName is the partial key of Dependents.
Also, DLocation may be treated as a multivalued attribute.

Step 3: Identify the Relationship Sets

In this step we need to find all the meaningful relationship sets among possible entity sets. This
step is very tricky, as redundant relationships may lead to complicated design and in turn a bad
implementation.

Example: Let us show below what the possible relationship sets are:
1. Employees and Departments WorksFor
2. Employees and Departments Manages
3. Departments and Projects Controls
4. Projects and Employees WorksOn

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