Mapping ER Models to Relational Schemas - unibz
Introduction to Database Systems Mapping ER Models to Relational Schemas
Werner Nutt
1
Conceptual and Logical Design
Conceptual Model:
name PRODUCT
price
BUYS
PERSON
name
ssn
Relational Model:
2
Mapping an E-R Diagram to a Relational Schema
We cannot store date in an ER schema (there are no ER database management systems)
? We have to translate our ER schema into a relational schema
? What does "translation" mean?
3
Translation: Principles
? Maps ? ER schemas to relational schemas ? ER instances to relational instances
? Ideally, the mapping should ? be one-to-one in both directions ? not lose any information
? Difficulties: ? what to do with ER-instances that have identical attribute values, but consist of different entities? ? in which way do we want to preserve information?
4
Mapping Entity Types to Relations
? For every entity type create a relation ? Every atomic attribute of the entity type becomes a relation attribute ? Composite attributes: include all the atomic attributes ? Derived attributes are not included
(but remember their derivation rules) ? Relation instances are subsets of the cross product
of the domains of the attributes ? Attributes of the entity key make up the primary key of the relation
given
family
name
STUDENT
studno
no. of students
courseno equip
COURSE
subject 5
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