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**Transform an ER Model into a Relational Database Schema**

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**2 into 1 won’t go? ER model has 2 major concepts**

Entities Relationships Relational model has 1 major concept Relation (table) There are general rules for translation good implementations come from these and experience/inventiveness inventiveness requires clear understanding of the relational model

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**How we do it Entities Relationships all become relations (tables)**

some become relations (tables) some are implemented by use of PK, FK some need additional coding using DBMS facilities using application code if necessary we know which by their cardinality signatures

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**Notation Primary key attribute(s): underline & bold**

Foreign key attributes: *

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Entities to Relations Start off by representing each entity class as a relation Add the attributes Indicate primary key Operator (Code, Name, Tel) Tour (Id, Start, Visiting)

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**Relationships to Relations Three Simple Cases**

B A 1..1 0..* One to many Optional on the “many side” Many to many Optional on both sides 1:N Optional on both sides

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Simple Case 1 - example

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Simple Case 2 - example

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Simple Case 2 - comments The existence of a tuple in the “intersection” relation is the relationship instance The key is joint because a student can only take a module once SID as PK would let a student do only 1 module CODE as PK would let a module have only 1 student

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Simple Case 3 - example

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Simple Case 3 - comments Again, the existence of a tuple in the “intersection” relation is the relationship instance The intersection PK is only one FK (student) SID is PK so each student can have max 1 Sponsoring CO not PK, Sponsor could have many Sponsorings

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**Relationships to Relations Two problem cases**

1..1 1..* One to many Mandatory on both sides B A 0..* 1..* Many to many Mandatory on one sides

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**Problem Case 1 - example Can do no better than the optional case**

Module Lecturer 1..1 1..* Can do no better than the optional case Plant the key of the ‘one’ side in the ‘many’ side

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**Problem case 1 - comments**

How can we ensure that every instance of A is involved in at least one relationship with a B? i.e. every A appears in B Cannot enforce it Can check if rule is obeyed (rel. algebra) A[A] == B[A] Can query for As not found in B could query for operators not found in tours could list lecturers not teaching

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**Problem case 2 Can do no better than the optional case R(A*, B*)**

0..* 1..* Many to many Mandatory on one of the sides Can do no better than the optional case Plant both keys in a new relation with joint PK R(A*, B*) A(A, …) B(B, …)

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**Problem case 2 - comments**

How can we ensure that every instance of A (every A) is involved in at least one relationship with a B? (same question) Cannot enforce it (same problem!) Every A must appear in R at last once Can check if rule is obeyed (rel. algebra) A[A] == R[A] Can query for As not found in R etc.

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**Problem cases - general**

These cases are the less common ones Often the constraints cannot be implemented for all time modules and students before registration? Often left unimplemented but with a mechanism to list breaches a query, run regularly or on demand Enforcing participation may just not be important

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**Comprehensive list of signatures**

Reference material

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**1:N Relationships A(A,…) B(B,…) R(A*,B*) A(A,…) B(B, A*,…) A(A,…)**

0..1 1..* R(A*,B*) A(A,...) B(B, A*,...) B A 1..1 1..*

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**Binary (M:N) Relationships**

A(A,…) B(B,…) R(A*,B*) A(A,…) B(B,…) R(A*,B*) B A 1..* 0..* A(A,…) B(B,...) B A 0..* 1..* R(A*,B*) A(A,...) B(B,...) B A 1..* R(A*,B*)

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**Binary (1:1) Relationships**

R(A*,B*) or A(A,…) B(B,…) A(A,…) B(B, A*…) No Duplicates c.f. above A B Not Null & No Duplicates A(A,…) B(B, A*…) B A 1..1

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