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# E/R to relational mapping - PowerPoint PPT Presentation

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

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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

• 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

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