Converting an E-R Model to a Relational Model:

The E-R model should be converted to a model which can be used directly in a database. Therefore, we need to convert the E-R model to a relational model. The relational model consists of relations, tuples, and attributes. A relation is a two dimensional table which is made up of tuples. A tuple in the relational model is a row in a database. It is made up of one or more attributes (or columns in the database model). Attribute is a characteristic or a property of the relation contained in a tuple. Table gives the different terms which are used by different models.

Diffrent terms used in diffrent model

Converting an entity into a relation

In most of the E-R models, entities and weak entities get easily converted into relations. Each entity gets directly converted to a relation. The entity instances in the entity become tuples in the relation. The attributes in the entity become the attributes in the relation. The identifier in the entity becomes a key in the relation. If a weak entity is not dependent on the identifier (key), then it should be treated like any other entity. But if it is dependent, then the key or the identifier of the parent relation is copied into the dependent relation. The key is called a foreign key.

Representing relationships

In case of 1:1 relationships, the key of one relation is stored in other relation. In case of 1:N relationships, the parent entity lies on the ‘1' side of the relation while the child entity lies on the ‘N' or ‘Many' side of the relation. The key of the parent entity is copied into the child relation. M:N relationships cannot be directly represented in relations. To represent them, a third relation, called the intersection relation should be introduced. The keys from the original two relations should be copied into the intersection relation.
Representing recursive relationships: A recursive relationship takes place between instances of a single type of entity. Recursive relationships are also called unary relationships. Let us consider the example of a car. Various parts or components should be assembled to roll out a complete car. Therefore, a car is made up of one or many other parts or components. Also, a part or a component might be made up of one or more parts or components. This results in a recursive 1:N relationship. Other forms include 1:1 and N:M recursive relationships. If a student teaches one and only one other student, it forms a recursive 1:1 relationship. However, if the student teaches one or many other students and also that student is taught by one or many other students, then it forms a recursive N:M relationship. These can be represented directly in the form of a relationship by adding another copy of the key in the relation.

Representing ternary relationships: A ternary relationship takes place when there exists binary or simultaneous relationships between three types of entities. To represent these in a relationship, they should be first rewritten as numerous binary relationships. After rewriting them, steps similar to those followed for converting entities into relations should be followed. Let us take an example of a supplier who sends components to a warehouse. Here, there are three entities - the supplier, the components, and the warehouse. In order to represent this as a relationship, binary relationships have to be first established between these entities. Binary relationships should be established between the suppliers and the components, the suppliers and the warehouse, and the warehouse and the components.

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