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E/R to relational mappingPowerPoint Presentation

E/R to relational mapping

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### E/R to relational mapping

- E/R to relational mapping algorithm – overview
- Mapping algorithm step 1-7
- Steen Jensen, autumn 2013

E/R to relational mapping algorithm - overview

- The next slides shows an algorithm for converting an E/R diagram to a relational model
- The algorithm contains 7 steps:
- Mapping of regular entity types
- Mapping of weak entity types
- Mapping of binary 1:1 relationship types
- Mapping of binary 1:N relationship types
- Mapping of binary M:N relationship types
- Mapping of multivalued attributes
- Mapping of N-ary relationship types

E/R to relational mapping algorithm – base example

The steps will be based upon figure 7.1 from Elmasri chapter 7

E/R to relational mapping algorithm – step 1- mapping of regular entity types

- For each regular (strong) entity type create a relation, that includes all simple attributes
- Choose one of the attributes as a primary key
- See a) in the below figure

E/R to relational mapping algorithm – step 2- mapping of weak entity types

- For each weak entity type create a relation, that includes all simple attributes
- Include as foreign key the primary key(s) of the relation(s) corresponding to the ownerentity type(s)
- The primary key is the combination of the foreign key and the partial key of the weak entity
- See b) in the below figure

E/R to relational mapping algorithm – step 3- mapping of binary 1:1 relationship types

- Include a foreign key in one of the entities taking part of the relationship (pick either end)
- Also include any attributes from the relationship
- See the below figure – here the entity type Department is chosen (relationship Manages)

E/R to relational mapping algorithm – step 4- mapping of binary 1:N relationship types

- Include a foreign key in the N-side of the relationship
- Also include any attributes from the relationship
- See the below figure – the relationships Works_for, Controls and Supervision has been implemented)

E/R to relational mapping algorithm – step 5- mapping of binary M:N relationship types

- Each M:N relationship is converted into a relation with a combined primary key (the primary key from both participating entities)
- Also include any attributes from the relationship
- See c) in the below figure – the relationship Works_onhas been implemented

E/R to relational mapping algorithm – step 6- mapping of multivalued attributes

- Each multivalued attribute is converted into a relation with a combined primary key: the attribute itself + foreign key
- For each value a tuple will exist in the new relation
- See d) in the below figure – the multivalued attribute Locations has been implemented

E/R to relational mapping algorithm – step 7- mapping of N-ary relationship types

Figure3.17 from chapt. 3 in Elmasri

Figure7.4 from chapt. 7 in Elmasri

- Each N-ary relationship is converted into a relation with a combined primary key (the primary keys from all participating entities)
- Also include any attributes from the relationship

Assignment: Map E/R diagram for Amazon to a relational model

Continue with your solution for Amazon – the E/R diagram

Follow the steps described in the slides and convert the E/R diagram into a relational model

Start today and continue at home, so you can present a solution next time in class – make a Word document with the relational models and upload it to the folder called Relational models for Amazonin studie.rhs.dk

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