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ER Diagrams 1

The overall logical structure of a database can be expressed graphically by an ER diagram . It consists of
rectangles - which represents entity sets ,
ellipses - which represents attributes,
diamonds - which represents relationship among entity sets and
lines which links attributes to entity sets, attributes to relationship sets and entity sets to relationship. Each component is labeled with the entity or relationship or attribute it represents.
To distinguish among the four types of mapping cardinalities type, we shall draw either a directed or undirected line segment between the relationship set and the entity set.


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ER Diagrams to Table

REDUCING E-R DIAGRAM TO TABLE - A database which conforms to an E R diagram can be represented by collection of tables .For each entity set and for each relationship set in the database, we will create unique tables, which is assigned the name of the corresponding entity set or relationship sets . Each table has a no. of columns which have unique names. Each row in the table corresponds to an entity or a relationship.

REPRESENTATION OF STRONG ENTITY SET -Let E be a strong entity set with descriptive attributes a1, a2....aN . We represent this entity by table called E with N distinct columns, each of which corresponds to one of the attributes of E.

REPRESENTATION OF RELATIONSHIP SET - Let R be a relation ship set involving entity set E1,E2....En Let attribute(R) consists of 'm' attributes We can represent this relation ship set by a table called R with m distinct columns, each of which corresponds to one of the attributes in attribute (R) plus the primary key of E1..En.