1. Translating ER Schema to Relational Model
Instructor: Mohamed Eltabakh
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2. First: Check Your Oracle Account
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3. Translating ER Schema to Relational Schema
Primary keys allow entity sets and relationship sets to be expressed uniformly as relational schemas
Generally, each relational schema will have
Number of columns corresponding to the number of attributes in ERD
Column names that correspond to the attribute names
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4. More optimizations will come …
Basic Mapping
Simple algorithm covers base the cases, Idea:
Each entity set  separate relation
Each relationship type  separate relation
Define the primary keys as discussed in the ER Model
More optimizations will come …
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5. Example 1 Loan (load_number, amount)
Customer (customer_id, customer_name, customer_street, customer_city)
Borrower (customer_id, load_number) – Many-to-Many Relationship
FOREIGN KEY Borrower (customer_id) REFERENCES Customer (customer_id)
FOREIGN KEY Borrower (loan_number) REFERENCES Loan (loan_number)
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6. Example 2 FOREIGN KEY Offers(dNumber) REFERENCES Dept(dNumber)
Course
offers
Dept (dNumber, dName)
Course (cNumber, cName)
Offers (dNumber, cNumber) -- One-to-Many Relationship from Dept to Course
FOREIGN KEY Offers(dNumber) REFERENCES Dept(dNumber)
FOREIGN KEY Offers (cNumber) REFERENCES Course(cNumber)
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7. Example 3 Product (pName, pNumber) Supplier (sName, sLoc)
Consumer(cName, cLoc)
Supply (sName, cName, pName, price, qty)
FOREIGN KEY Supply(sName) REFERENCES Supplier(sName)
FOREIGN KEY Supply (pName) REFERENCES Product(pName)
FOREIGN KEY Supply (cName) REFERENCES Consumer(cName)
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8. Example 4 Part(pNumber, pName)
Contains(super_pNumber, sub_pNumber, quantity)
FOREIGN KEY Contains (super_pNumber) REFERENCES Part (pNumber)
FOREIGN KEY Contains (sub_pNumber) REFERENCES Part (pNumber)
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9. Refinement I: Weak Entity Sets
Weak entity set does not have its own key
It must relate to the identifying entity set via a total, one-to-many relationship set from the identifying to the weak entity set
A weak entity set is mapped to a relation with all its attributes + the key(s) of the identifying entity set(s)
Primary key of the new relation is the:
Identifying key(s) from identifying entity set(s), Plus
Discriminator of the weak entity set
Supporting relationship is not mapped in the relation model
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10. Example 5 Dept(dNumber, dName) Course(dNumber, cNumber, cName)
FOREIGN KEY Course(dNumber) REFERENCES Dept(dNumber)
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11. Refinement II: One-to-Many & Many-to-One Cardinalities
Many-to-one and one-to-many relationship sets can be represented by adding an extra attribute to the “many” side, containing the primary key of the “one” side
This transferred primary key becomes a foreign key
The relationship itself is not mapped to the relational model
Any attributes on the relationship go to the “Many” side
Course
offers
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12. Compare this with Example 2 (this is a better representation)
Course
offers
Dept (dNumber, dName)
Course (cNumber, dnumber, cName)
FOREIGN KEY Course(dNumber) REFERENCES Dept(dNumber)
Compare this with Example 2 (this is a better representation)
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13. Example 7
Part(pNumber, pName)
SubPart(sub_pNumber, super_pNumber, quantity)
FOREIGN KEY SubPart(super_pNumber) REFERENCES Part (pNumber)
FOREIGN KEY SubPart(sub_pNumber) REFERENCES Part (pNumber)
Compare this with Example 4(Here the primary key of subPart is stronger)
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14. Example 8 FOREIGN KEY Course(dNumber) REFERENCES Dept(dNumber)
offers
Open head (one and must be one)
Dept (dNumber, dName)
Course (cNumber, dnumber, cName)
FOREIGN KEY Course(dNumber) REFERENCES Dept(dNumber)
Compare this with Example 6
-- In Example 6: Course.dnumber can be null
--In Example 8: Course.dnumber cannot be null
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15. Refinement III: One-to-One Cardinalities
One-to-one relationship sets can be represented by adding the primary key of either sides to the other side
This transferred primary key becomes a foreign key
The relationship itself is not mapped to the relational model
Any attributes on the relationship go to the side receiving the transferred primary key
Player
pName
pID
Storage area
Number
Location
size
owns
StartDate
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16. Example 9 Player(pID, pNumber)
pName
pID
Storage area
Number
Location
size
owns
StartDate
Player(pID, pNumber)
StorageArea(Number, pID, startDate, Location, size)
FOREIGN KEY StorageArea(pID) REFERENCES Player(pID)
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17. Refinement IV: Composite & Derived Attributes
sNum
sName
Student
sAge
address
street
city
state
Mapping strategy (Composite): Include an attribute for every primitive component of the composite attribute in the entity
Mapping strategy (Derived): Mapped as is (enforced later using triggers)
Student(sNum, sName, sAge, street, city, address)
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18. Refinement V: Multi-valued Attributes
sNum
sName
Student
major
sAge
address
street
city
state
Mapping strategy:
Represented as a relation by itself.
The primary key of the relation = Attribute + the primary key of the main entity set
Student(sNum, sName, sAge, street, city, address)
StudentMajor(sNum, major)
FOREIGN KEY StudentMajor (sNum) REFERENCES Student (sNum)
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19. Refinement VI: ISA Relationships
ISA is a one-to-one relationship BUT the sub-class entity sets inherit attributes from the super-class entity set
That is why it does not follow the one-to-one rules
Basically many ways for the mapping depending on whether it is total vs. partial and overlapping vs. disjoint
Super-class key is always the primary key

20. ISA Relationship : Method 1 (Relation for each Entity Set)
Student (sNumber, sName)
UGStudent (sNumber, year)
GradStudent (sNumber, program)
PRIMARY KEY (Student) = <sNumber>
PRIMARY KEY (UGStudent) = <sNumber>
PRIMARY KEY (GradStudent) = <sNumber>
FOREIGN KEY UGStudent (sNumber)
REFERENCES Student (sNumber)
An UGStudent will be represented in both Student relation as well as UGStudent relation
A GStudent will be represented in both Student relation as well as GStudent relation

21. ISA Relationship : Method 2 (One Relation for All)
Student (sNumber, sName, year, program)
PRIMARY KEY (Student) = <sNumber>
Note: There will be null values in the relation.
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22. ISA Relationship : Method 3 (Relations only for SubClasses)
Any student will be represented in only one or possibly both relations as appropriate
Assumes total relationship
UGStudent (sNumber, sName, year)
GradStudent (sNumber, sName, program)
PRIMARY KEY (UGStudent) = <sNumber>
PRIMARY KEY (GradStudent) = <sNumber>
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23. ISA Relationship : Method 4 (Relation for each combination)
Any student will be represented in only one of the relations as appropriate.
Student (sNumber, sName)
UGStudent (sNumber, sName, year)
GradStudent (sNumber, sName, program)
UGGradStudent (sNumber, sName, year, program)
PRIMARY KEY (Student) = <sNumber>
PRIMARY KEY (UGStudent) = <sNumber>
PRIMARY KEY (GradStudent) = <sNumber>
PRIMARY KEY (UGGradStudent) = <sNumber>

24. Mapping from ER model to Relational model: Summary
Basic algorithm covers the main cases
Refinement I : Weak Entity Sets
Refinement II : One-to-Many Relationships
Refinement III : One-to-One Relationships
Refinement IV : Composite & Derived Attributes
Refinement V : Multi-Valued Attributes
Refinement VI : ISA Relationships
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25. What about an Exercise
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