_Practical 7
Que-1 . Explain Data object, attribute and relationship
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Data object:
A data object is a representation of any fundamental composite information of system. A data object ha a number of different properties or attributes which distinguish it from another object.
Object type is the general description of all objects which has same attribute.
A data object can be an external entity, a thing, an occurrence or event, a role, an organization unit, a place, or a structure.
Data attribute:
Attribute defines the properties of a data object. Attribute is a characteristics of object or entity that is meaningful and interest to organization system.
if we continue out example of college, student has some property: Student_name, Student_ID, Address.
College has attribute: College_ID, College_Name, Address, Afflicated Uni.
attribute is illustrated inside oval shape.
One or more off the attribute must be defined as an identifier-- that is, the identifier attribute becomes a key when we can an instance of the data object.
relationship :
Data objects are connected to each other in different way. Relationship is a reason for associating two entities.
Generally this relationship is called as binary relationship because in general there are two entities are related to each other.
It is important to note that object/relationship pairs are bidirectional. That is, they can be read in either direction. Relationship is shown by diamond shape with relationship name joined with line.
Que-2 . Explain cardinality and Modality.
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Cardinality:-
we have defined a set of entities and represent the entity- relationship pairs.
but a simple pair does not provide enough information for software engineering purposes.
The data model must be capable of representing the number of occurrences of objects in a given relationship.
we must understand how many occurrences of entity X are related to how many occurrences of entity Y. This lead to a data modelling concept called as cardinality, defines " the maximum number of objects that can participate in a relationship".
One-to-one(1:1):--An occurrence of entity A can be relate to one and only one occurrence of entity B and vice-versa.example:-- one college have only one principal.
One-to-many(1:M):--One occurrence of entity A can be relate to one or many occurrences of entity B, but an occurrences of B can be relate to only one occurrence of A.example:-- many students are enrolled to only one college.
Many-to-many(M:M):--An occurrence of entity A can be relate to one or many occurrence of entity B and vice-versa.
example:-- many student can registered for many subjects.
Modality:-
Modality means a classification of relationships on the basis of weather they claim necessity, possibility or impossibility.
Modality is form of cardinality. The modality of a relationship is 0 if there is no explicit need for the relationship or the relationship is 0 or the relationship is optional.
The modality is 1 if an occurrence of the relationship is mandatory.
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