1. ER-to-Relational Mapping
Database Management Systems, 3rd ed., Ramakrishnan and Gehrke, Chapter 3

2. Regular Entity Set LName FName SSN MidInit Sex Employees Address
Create relation for the entity set; include all attributes. Choose one of the key attributes as the PK.
Employees
Address
Salary
Employee (SSN, FName, MidInit, LName, Sex, Address, Salary)

3. Relationship Set – No Key or Participation Constraints
Name
Since
DName
Create relation for relationship set. Include as FKs the PKs of each participating entity set. Include descriptive attributes of relationship set. Relation’s PK is collection of PK attributes from participating entity sets.
SSN
Lot
Budget
DID
WorksIn2
Employees
Departments
Address
Locations
Create relations for the entity sets:
Employees (SSN, Name, Lot)
Departments (DID, DName, Budget)
Locations (Address, Capacity)
and a relation for the relationship set:
WorksIn2 (SSN, DID, Address, Since)
SSN is FK that references Employees.
DID is FK that references Departments.
Address is FK that references Locations.
Capacity
Figure 3.10

4. Roles
supervisor
Employees
ReportsTo
SSN
Name
Lot
To represent the ReportsTo relationship set, create meaningful attribute names for SSN to represent the two different roles played by Employees.
subordinate
Create relation for the entity set:
Employees (SSN, Name, Lot)
and relation for the relationship set:
ReportsTo (SupervisorSSN, SubordinateSSN)
SupervisorSSN is FK that references SSN in Employees.
SubordinateSSN is FK that references SSN in Employees.
Figure 3.11

5. Relationship Set – Key Constraint Approach 1
Suppose relationship set has n participating entity sets of which m are linked via arrows. The key for any one of those m entity sets is a candidate key for relation representing relationship set.
Name
Since
DName
SSN
Lot
Budget
DID
Manages
Employees
Departments
Figure 3.12
Create relations for the entity sets:
Employees (SSN, Name, Lot)
Departments (DID, DName, DBudget)
and a relation for the relationship set:
Manages (SSN, DID, Since)
SSN is FK that references Employees.
DID is FK that references Departments.
Why not have (SSN, DID) as the PK for Manages?

6. Relationship Set – Key Constraint Approach 2
Do not define separate relation for relationship set. Relationship is represented in relation corresponding to entity set with the key.
Name
Since
DName
SSN
Lot
Budget
DID
Manages
Employees
Departments
Figure 3.12
Create relations for the entity sets. Add SSN to Departments to represent relationship set; also include the relationship’s descriptive attribute.
Employees (SSN, Name, Lot)
Departments (DID, DName, DBudget, SSN, Since)
SSN is FK that references Employees.
Note: Book renamed the Departments relation DeptMgr.

7. Relationship Set – Participation Constraint
Name
Since
DName
SSN
Lot
Budget
DID
Manages
Employees
Departments
Each department has exactly one manager:
Participation constraint: Each department must have at least 1 manager.
Key constraint: Each department may have at most 1 manager.
WorksIn
Figure 3.13
Since

8. Participation Constraint (continued)
Note: We’re using the 2nd approach for representing the key constraint. Why would the 1st approach not work in this case?
Employees
Manages
SSN
Name
Lot
DID
DName
Budget
Since
Departments
WorksIn
Employees (SSN, Name, Lot)
DeptMgr (DID, DName, Budget, SSN, Since)
SSN is FK that references Employees;
cannot be null.
WorksIn (SSN, DID, Since)
SSN is FK that references Employees; every SSN
in Employees must appear in WorksIn.
DID is FK that references DeptMgr; every DID in
DeptMgr must appear in WorksIn.
Figure 3.13

9. Weak Entity Set
Name
Create relation for weak entity set. Include as FKs the PKs of the owner entity set. PK for new relation is owner’s PK and weak entity set’s partial key. Use 2nd approach given earlier to represent 1:N relationship set between owner and weak entity set. Specify “on delete cascade” to indicate existence dependence.
Cost
SSN
Lot
PName
Age
Employees
Policy
Dependents
Figure 3.14
Define relation for Employees:
Employees (SSN, Name, Lot)
and relation for Dependents entity set plus Policy relationship set:
DepPolicy (SSN, PName, Age, Cost)
SSN is FK that references Employees; on delete cascade.

10. Class Hierarchy Approach 1
Name
Lot
SSN
Employee
Map each entity set to a relation. For each subclass entity set, include the PK of the superclass. The subclass’s PK is the same as the superclass’s PK and is a FK; specify “on delete cascade”.
ISA
HoursWorked
HourlyEmps
ContractEmps
Employee (SSN, Name, Lot)
HourlyEmps (SSN, HourlyWages,HoursWorked)
SSN is FK that references Employee; on delete
cascade.
ContractEmps (SSN, ContractID)
HourlyWages
ContractID
Figure 3.15

11. Class Hierarchy Approach 2
Name
Lot
SSN
Employee
Map only the subclass entity sets to relations. For each subclass entity set, include the PK of the superclass.
ISA
HoursWorked
HourlyEmps
ContractEmps
HourlyWages
ContractID
HourlyEmps (SSN, Name, Lot, HourlyWages, HoursWorked)
ContractEmps (SSN, Name, Lot, ContractID)
Figure 3.15

12. Aggregation Name Lot SSN Employees Employees (SSN, Name, Lot) Until
Projects
SSN
Name
Lot
Monitors
Until
Sponsors
Departments
Since
PID
StartedOn
PBudget
DID
DName
Budget
Employees (SSN, Name, Lot)
Projects (PID. StartedOn, PBudget)
Departments (DID, DName, Budget)
Sponsors (PID, DID, Since)
Every PID in Projects must appear in
Sponsors.
Monitors (SSN, DID, PID, Until)
Note that the relation representing the Monitors relationship set includes the key attributes of the Sponsors relation and the descriptive attributes of the Monitors relationship set.
Figure 3.16