What do you understand by a strong entity and a weak entity? Provide example.
A strong entity is complete by itself and is not dependent on any other entity type. EXAMPLE customer(customerID, name,surname)
A weak entity is dependent on a strong entity to ensure its existence. Adress(addressid, adressName, customerid)
i. What do you mean by a one-to-many relationship between Room and Class table?
a) One room may have many classes
b) One class can be run in many rooms
c) Many classes can be run in many rooms
d) Many rooms may have many classes
ii. When an entity has a relationship to itself, we have a(n) ________.
a) supertype/subtype relationship
b) archetype/instance relationship
c) recursive relationship
d) Both A and C are correct
iii. Supertype/subtype entities are said to have a(n) ________ relationship.
iv. An entity whose identifier includes the identifier of another entity is a(n) ________.
a) strong entity
b) weak entity
c) optional entity
d) required entity
v. A circle across the relationship line near an entity indicates ________.
a) a maximum cardinality of “zero”
b) a maximum cardinality of “one”
c) a minimum cardinality of “optional”
d) a minimum cardinality of “required”
Give examples of unary, binary and ternary relationships.
Unary= A teacher who teaches student
Binary = Journalist writes an article.
Ternary= University might need to record which teachers taught which subjects in which courses
What are multi-valued attributes? Give examples.
Multi-valued attributes is an attributes that has more than one value. Examples can be student’s phone number and email address.
For each of the following pairs of rules, identify two entity types and one relationship. State the cardinality and existence of the relationship in each case. If you don’t think enough information is available to define either of these, then state an assumption that makes it clear. Also draw the ER diagram for given statements.
1. A department employs many persons. A person is employed by, at most, one department.
2. A manager manages, at most, one department. A department is managed by, at most, one manager.
3. An author may write many books. A book may be written by many authors.
4. A team consists of many players. A player plays for only one team.
5. A lecturer teaches, at most, one course. A course is taught by exactly one lecturer.
6. A purchase order may be for many products. A product may appear on many purchase orders.
7. A customer may submit many orders. An order is for exactly one customer.
Prepare an ERD for a real estate firm that lists property for sale. Also prepare a definition for each entity type and attribute on your diagram. The following details describe this organisation:
The firm has a number of sales offices in several states. Attributes of sales office include OfficeID (identifier) and Location.
Each sales office is assigned one or more employees. Attributes of employee include EmployeeID (identifier) and EmployeeName. An employee must be assigned to only one sales office.
The firm lists property for sale. Attributes of property include PropertyID (identifier) and Location.
Each unit of property must be listed with one and only one of the sales offices. A sales office may have any number of properties listed or may have no properties listed.
Each property has one or more owners. Attributes of owners are OwnerID (identifier) and OwnerName. An owner may own one or more units of property. An attribute of the relationship between property and owner is PercentOwned.
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