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Translating ER Schema to Relational Model

Translating ER Schema to Relational Model. Simple Algorithm. Entity type E  Relation E’ Attribute of E  A ttribute of E’ Key for E  Primary Key for E’. Simple Algorithm. For relationship type R between E 1 , E 2 , …, E n Create separate relation R’ Attributes of R’ are :

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Translating ER Schema to Relational Model

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  1. Translating ER Schema to Relational Model CS3431

  2. Simple Algorithm • Entity type E  Relation E’ • Attribute of E  Attribute of E’ • Key for E  Primary Key for E’ cs3431

  3. Simple Algorithm • For relationship type R between E1, E2, …, En • Create separate relation R’ • Attributes of R’ are : • primary keys of E1, E2, …, En and attributes of R • Primary Key for R’ is defined as: • Primary key for R’ = primary keys for E1, E2, …, En • Foreign Keys: • Define “appropriate” foreign keys from R’ to E1, E2, …, En cs3431

  4. Simple algorithm: Example 1 Person (pNumber, pName) Dept (dNumber, dName) WorksFor (pNumber, dNumber, years) PRIMARY KEY (Person) = <pNumber> PRIMARY KEY (Dept) = <dNumber> PRIMARY KEY (WorksFor) = <pNumber, dNumber> FOREIGN KEY WorksFor (pNumber) REFERENCES Person (pNumber) FOREIGN KEY WorksFor (dNumber) REFERENCES Dept (dNumber) cs3431

  5. Simple Algorithm: Example 2 Supplier (sName, sLoc) Consumer (cName, cLoc) Product (pName, pNumber) Supply (supplier, consumer, product, price, qty) PRIMARY Key (Supplier) = <sName> PRIMARY Key (Consumer) = <cName> PRIMARY Key (Product) = <pName> PRIMARY Key (Supply) = <supplier, consumer, product> FOREIGN KEY Supply (supplier) REFERENCES Supplier (sName) FOREIGN KEY Supply (consumer) REFERENCES Consumer (cName) FOREIGN KEY Supply (product) REFERENCES Product (pName) cs3431

  6. Simple Algorithm: Example 3 Part (pName, pNumber) Contains (superPart, subPart, quantity) PRIMARY KEY (Part) = <pNumber> PRIMARY KEY (Contains) = <subPart,superPart> FOREIGN KEY Contains (superPart) REFERENCES Part (pNumber) FOREIGN KEY Contains (subPart) REFERENCES Part (pNumber) cs3431

  7. Refinement for Simple Mapping • Primary Key for R’ is defined as: • If the maximum cardinality of any Ei is 1, primary key for R’ = primary key for Ei cs3431

  8. Simple Algorithm: Example 3 Part (pName, pNumber) Contains (superPart, subPart, quantity) PRIMARY KEY (Part) = <pNumber> PRIMARY KEY (Contains) = <subPart> FOREIGN KEY Contains (superPart) REFERENCES Part (pNumber) FOREIGN KEY Contains (subPart) REFERENCES Part (pNumber) cs3431

  9. Summary So Far • Simple algorithm covers base case • Idea: “Each relationship type = separate relation” NEXT: • Let’s consider constraints • Let’s reduce number of relations cs3431

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