Database Design Strategies

In Chen ERD, multivalued attributes are represented by:

• rectangle

• double ellipse

• dashed ellipse

• solid ellipse

In Chen ERD, what symbol is used to represent a weak entity?

• double rectangle

• double ellipse

• dashed ellipse

• solid ellipse

Which symbol is used to represent a derived attribute in Chen ERD?

• double rectangle

• double ellipse

• dashed ellipse

• solid ellipse

Given the basic ER diagram and relational model, which of the following is INCORRECT?

• An attribute of an entity can have more than one value

• An attribute of an entity can be composite

• In a row of a relational table, an attribute can have exactly one value or a NULL value

• In a row of relational table, an attribute can have more than one value

Which of the following possible relations will NOT hold if the above ERD is mapped into a relation model ?

• Person (NID, Name)

• Qualification (NID, ExamID, QualifiedDate)

• Exam (ExamID, NID, ExamName)

• Exam (ExamID, ExamName)

Answer explanation

The relation Exam (ExamID, NID, ExamName) violates the ERD mapping as it does not match the structure of the Exam entity in the ERD.

Consider the given ERD. We create tables a and b, each of which have a primary key column named “id”. (Assume there are additional columns from attributes not shown.) What is the simplest way to convert the relationship between A and B?

• Create a column named “a_id” in table b, and make it a foreign key referencing table a.

• Create a column named “b_id” in table a, and make it a foreign key referencing table b.

• Create a cross-reference table, a_b, containing columns a_id and b_id as foreign keys referencing a and b respectively.

• Merge tables a and b into a new table.

Consider the given ERD. We create table r with primary key column id. What should table w look like?

• The table should have a column partial as primary key. Additionally, create a cross-reference table r_w.

• The table should have columns partial and r_id. The primary key is partial. Add a foreign key constraint on r_id referencing r.

• The table should have columns partial and r_id. The primary key is a composite of r_id and partial. Add a foreign key constraint on r_id referencing r.

• The table should have columns partial and r_id. The primary key is r_id. Add a foreign key constraint on r_id referencing r.

In the logical design, what is the process BEFORE a data dictionary is developed?

Given the data dictionary of a club table, what can we say about relationship between CLUB and TEACHER?

• TEACHER is optional for CLUB

• TEACHER is mandatory for CLUB

• CLUB is optional for TEACHER

• CLUB is mandatory for TEACHER

What is the purpose of a weak entity in an ERD?

• To represent an entity that does not have a primary key.

• To represent an entity that is dependent on another entity.

• To represent an entity that has a composite key.

• To represent an entity that is optional in a relationship.