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Published byShona Stevenson Modified over 9 years ago
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Conversion from ER diagram to relational model
ER is a visual model The relational model is a matemathical model Conversion from ER diagram to relational model
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Database development as part of systems development
Analysis phase Draw an ER-diagram Design phase Convert ER-diagram to relational model. Check if the relations are sufficiently normalized. Realization phase Create tables in a DBMS. Conversion from ER diagram to relational model
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Procedure for conversion from ER to relational model
Strong entity types have their own relation with all the single-valued attributes. composite attributes: Simple attributes (“leaves”) in the relations. primary key transferred from ER to the relation. Conversion from ER diagram to relational model
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Converting weak entities
Weak entity types have their own relation with all single-valued attributes partial key transferred from ER to the relation foreign key to referring to the “strong” entity (identifying relationship). primary key = partial key + foreign key to “strong” entity. Conversion from ER diagram to relational model
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Binary relationships 1:1
Binary 1:1 relationships (rare) data from the relationship (if any) integrated into one of the related entities [your choice]. one of the related relations gets a foreign key to the other [your choice: easy to find a “way” from on to the other, but not vice versa] 1:1 relations are rare in ER-diagrams {they are usually modeled as a single entity } Conversion from ER diagram to relational model
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Binary relationships 1:N
Binary relationships with cardinality 1:N data from the relationship (if any) integrated into the relation with the “N” The “N” relation has a foreign key to the “1” relation. Conversion from ER diagram to relational model
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Binary relationships N:M
Binary relationships with cardinality N:M Own relation with 2 foreign keys (1 for each of the relation entities) Date (like timestamps, etc.) on the relationships goes into the relation. Conversion from ER diagram to relational model
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Multi valued attributes
Own relation Foreign key to the “owning” relation. Conversion from ER diagram to relational model
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Conversion from ER diagram to relational model
N-ary relationships Relationships with N (more than 2) entities Own relation with N foreign keys (1 for each related entity) Conversion from ER diagram to relational model
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Conversion from ER diagram to relational model
Inheritance, page 295 ER-modeling allows inheritance between entity types. Problem No inheritance in the relational model. Solution Try to avoid inheritance in ER. Conversion from ER diagram to relational model
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