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Database Design - Structure

Conceptual model

Logical model

Physical model

- General (not attached to any model)- Not normalized- e.g. ER diagram, still has M:N relationships, composite & multivalued attributes

- Specific to data model, e.g. relational data model- Normalized- e.g. ER diagram, only has 1:1 & 1:M relationships- normalized relational schema

- Specific to DBMS- e.g. Oracle, CREATE TABLE statement

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Revisit the ERD

• Look for M:N relationships – change the diamond into rectangle symbol. Create a new relationship from entity1 to the newly created entity with 1:M connectivity & from the newly created entity to entity2 with M:1 connectivity.

• Look for multivalued attributes – create a new entity for the attribute. Create a key attribute for the new entity. Link the original entity with the newly created entity with 1:M connectivity if each value is unique to an instance, otherwise M:N

• Look for composite attributes – if each sub-attribute’s value is unique for each instance, then include each sub-attribute in the entity. Otherwise, create a new entity, and link the new entity to the original with connectivity M:1.

Mapping of ER Diagram to Relational Database

Schema

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Lesson Outcomes

By the end of the lesson, the students should be able to:• Explain some of the terminologies used in relational

database• Map an ER diagram to relational database schema.

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Relational Database is a collection of normalized relations with distinct relation names.

• A relation is a table with columns and rows.

• Normalized is defined as “conform to a norm or standard” – Merriam-Webster dict. In database, it means no unnecessary redundancies.

• An attribute is a named column/field of a relation.

• Domain is the set of allowable values for one or more attributes.

Relational Database Terminologies

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Unnormalized vs Normalized Relations

StudentID Name ProgCode Program Year

16227 Ali ICT Info & Comm. Technology 1

15113 Chong BIS Business Information Systems 2

15661 Peter ICT Info & Comm. Technology 2

16227 Ali BIS Business Information Systems 1

14355 Elaine EE Electrical & Electronic Engineering 3

14111 Kumar BIS Business Information Systems 3

StudentID Name ProgCode Year16227 Ali ICT 115113 Chong BIS 215661 Peter ICT 216227 Ali BIS 114355 Elaine EE 314111 Kumar BIS 3

ProgCode Program

ICT Info & Comm. Technology

BIS Business Information Systems

EE Electrical & Electronic Engineering

Unnormalized relation

Normalized relations

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• Tuple is a row/record of a relation.

• Degree is the number of attributes in a relation.

• Cardinality is the number of tuples in a relation.

© Pearson Education Limited 1995, 2005

Relational Database Terminologies

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Relational Database

tuple

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What is database schema?

• Database schema is the structure of the database when presented according to the database model chosen.

• For relational database, each entity in an ERD will become a relation.

• Each relation will have its relation schema• A relation schema shows the name of the relation along with

all of its attributes which include:– Relation name– Attribute names– Primary key– Foreign key – an attribute or set of attributes, within one relation that

matches primary key of some (possibly same) relation.

Properties of Relations

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• Relation name is distinct from all other relation names in relational schema.

• Each cell of relation contains exactly one atomic (single) value.

• Each attribute has a distinct name.

• Values of an attribute are all from the same domain.

• Each tuple is distinct; there are no duplicate tuples.

• Order of attributes and tuples in a relation has no significance.

© Pearson Education Limited 1995, 2005

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Steps to map ERD to relational database schema

• Begin with an entity which has 1 connectivity on its side.

• Write : Entity_name, (, attributes, ).

• Underline the primary key

• Continue with other entities (1 on their side)

ENT1 (attr1a, attr1b)

ENT1 (attr1a, attr1b)

ENT1

attr1a

rel11

attr1b

m

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Cont’d

• Next, select an entity with M on its side (in M:1 relationship)

• Write: Entity_name, (, attributes, the key of the 1 side, )

• Underline the primary key

• Circle the foreign key

• Continue with similar entities

ENT1

attr1a

rel11

ENT2 (attr2a, attr2b, attr1a)

attr1b

ENT2

attr2a

attr2b

m

ENT2 (attr2a, attr2b, attr1a)

ENT2 (attr2a, attr2b, attr1a)

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Cont’d

• Next, select the M:N relationship• Write: a relation name to represent the

relationship, (, p.k. of entity1, p.k. of entity2, attributes if any, )

• Underline the two keys, i.e., primary/composite key

• Circle each item of the composite key for foreign key

• Continue with similar relationships

ENT2

attr2a

rel2m

attr2b

ENT3

attr3a

attr3b

n

attr23aENT23 (attr2a, attr3a, attr23a)

ENT23 (attr2a, attr3a, attr23a)

ENT23 (attr2a, attr3a, attr23a)

ENT3 (attr3a, attr3b)

ENT2 (attr2a, attr2b)

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Cont’d - extra• If any of the entity has multivalued

attribute, then:

– create a new relation, create a key attribute and put the multivalued attribute in the new relation.

– Option 1: if each tuple of the new relation is only for one tuple of the original relation (1:1 or M:1), then

• delete the multivalued attribute from the original relation

• add the key of the original relation into the new relation. Circle the attribute for foreign key.

ENT1

attr1a

rel11

attr1b

m ENT1b(attr1ba, attr1b)

ENT1(attr1a)

ENT1b(attr1ba, attr1b, attr1a)

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Cont’d - extra

– Option 2: if each tuple of the new relation might be linked to many tuples of the original relation, vice-versa (M:N), then

• delete the multivalued attribute from the original relation.

• create another new relation. Fill it up with the keys from both the original relation and the created relation earlier.

ENT11b(attr1a, attr1ba)

ENT1(attr1a)

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Cont’d - extra

• If any of the entity has composite attribute, then:

– create a new relation, create a key attribute and list out all sub-attributes in the new relation.

– Replace the composite attribute of the original relation with the key of the new relation. Circle the attribute for foreign key

ENT1

attr1a

rel11

attr1b

m

attr1b1 attr1b2

ENT1b(attr1ba, attr1b1, attr1b2)

ENT1(attr1a, attr1ba)

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Lesson Summary

• Relational database schema represents the ERD in the relational data model

• Each entity will create a relation schema, so is the M:N relationship, multivalued attribute and composite attribute.