Evaluation of Four Languages for Specifying Conceptual Database Designs - High-Performance Web Databases: Design, Development, and Deployment - page 173

Exhibit 12-5. Equivalent Concepts and terminology between entity-relationship

and relational diagrams.

Entity-Relationship Concepts Relational Concepts

Entity set Table

Entity Row of a table

Attribute Column of a table

Unique identifier Primary key, candidate key

One-to-many relationship Foreign key constraint

Many-to-many relationship Two foreign keys referencing a linking table

Partial participation No NOT NULL declaration in a relationship for a foreign

key

Total participation NOT NULL declaration in a relationship for a foreign key

Weak entity Primary key is a composite key containing the primary

key of another table

Class hierarchy Two tables have the same primary key, one of which is

the foreign key of the contained table

In the example above, the entities Jones, Smith, and Lewis are members of
the Employee entity set, while entities Accounting and Sales are members
of the Department entity set.

Exhibit 6 shows the Employee entity set described using each of the four
design languages. The English text describes the Employee entity set as
having four attributes: Employee Identification, Employee Name, Address,
and Social Security Number. The entity-relationship diagram represents
the entity set as a rectangle labeled Employee with its attributes shown as
ovals labeled EmpId, EmpName, Address, and SocialSecurityNumber.
Attribute ovals are connected to the entity that they describe. The SQL
CREATE TABLE statement describes the same entity set as a table consist-
ing of four columns. The relational diagrams illustrate the Employee table
by showing a box with table name Employee at the top and column head-
ings EmpId, EmpName, Address, and SocialSecurityNumber at the bottom
of the box.

Key Constraint

Exhibit 7 extends the entity set presented in Exhibit 6 to include the con-
cept of a key constraint, which is used to enforce uniqueness among enti-
ties in an entity set. A key constraint specifies that no two entities in an
entity set can have the same value for the key attribute. (A key is a subset
of the attributes in the entity set.) In English, a constraint is specified by a
sentence in the form of “No two <entities> may have the same value for
<attribute>“ or “<attribute> uniquely identifies <entity>.” With entity-rela-
tionship diagrams, the attribute name of the unique identifier is under-
lined. In SQL, the clause PRIMARY KEY <attribute> identifies the unique