Various symbols used in the E-R diagrams are as follows:
Description of some symbols used in E-R diagrams:
An Entity is a basic element of a system identified by a set of attributes and has independent existence e.g. a Student, a faculty, a subject etc. in a college. An Entity type defines a set of entities that have the same attributes.
A relationship type R among n entity types E1,E2,… ,En defines a set of associations among these types.The Entity types which do not have key attributes of their own are called Weak Entity Types. For example: Entity Type ‘Dependent’ , related to an ‘Employee’. Which have attributes ‘Dependent name’, ‘Birth Date’, ‘Sex’ and ‘Relationship’. Two dependents of distinct employees may have the same values for all these attributes but they will still be distinct entities as they are linked to different Employee. Entities belonging to a weak entity type are identified by being related to specific entities from another entity type in combination with some of their attribute values. This other entity type is called identifying owner and the relationship type that relates the weak entity type to its owner is called identifying relationship type. For example in the above example the ‘Dependent’ is weak entity whose owner is ‘Employee’ and the relationship between ‘Dependent’ and ‘Employee’ will be a identifying relationship type.
The attributes which are keys to identify an entity in an entity set uniquely are called key attributes. For example attribute ‘Roll no.’ for an entity type ‘Student’ is a key attribute. Some attributes may have many values at the same time for an entity of entity set. For example the attribute ‘College Degrees’ which lists the name of degrees obtained by a person. It may have one value for a person but may have more than one value for others. Such attributes are called multi-valued attributes.
The attributes which are formed be combining smaller subparts are called composite attributes. For example an ‘Address’ attribute is formed by several sub parts like ‘Street’, ‘City’, ‘State’ and ‘Pin code’. The attributes which need not be stored with the entities and can be calculated from the values of other attributes which are stored are called derived attributes. For example if we have an attribute ‘Birth Date’ storing date of birth of a person then the attribute ‘Age’ need not be stored as it can be calculated whenever we access that entity. So ‘Age’ is a derived attribute. When all the entities of an entity set/ entity type have to participate in a particular relationship set/type, it is called total participation. Weak entities have total participation in identifying relationships with their identifying owners.
Degree of Relationship Set
The number of entity sets participating in a relationship is the degree of relationship Example of Unary Relationship. Only one entity set participates in the relationship.
Example of Binary relationship. Two entity sets participate in a relationship.
Example of ternary relationship. Three entity types participate in a relationship.
