Convert ER model into relation schema . There are well defined rules for this conversion. These are
- Converting strong Entity type
- Converting weak entity types
- Converting relationships
- Converting specialization
- Converting aggregation.
Converting strong entity types-
- Each entity become a table
- Each single valued attribute becomes a column
- Derived attribute are ignored.
- Composite attribute are represented by components.
- Multi valued attribute are represented by a separated table.
- The key attribute of the entity type becomes the primary key of the table.
Example of strong entity
Here address is a composite key, skill is a multi valued attribute
Converting weak entity types : Weak entity types are converted into a table of their own with the primary key of the strong entity acting as a foreign key in the table. This foreign key along with the key of the weak entity form the composite key of this table.
The way relationship are represented depends on the cardinality and the degree of the relationship. The possibility of cardinality are 1;1, 1:M, N:M, and the degree are Unary, binary, Ternary.
Converting specialization -: Generalization is concept of representing entity sets in the form of hierarchy.High level entity set is represented by table containing attribute common to lower level entity set. Low level entity set is represented by table containing column as primary key of higher level entity set and a specialized attribute corresponding to that lower level entity set.
Converting Aggregation -: If an entity set combines two entity sets with one relationship and this aggregation entity set is related with one more entity than. Each entity set is represented by one table.
The relationship between aggregation entity set is represented by one table. and the final table structure depends upon the relationship between the tables. The relationship between aggregation entity set and related entity is represented by one table containing primary key for each table.