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using the concepts defined previous we have identified that strong entities in college database are faculty student course and department this
aggregation one limitation of the e-r diagram is that they do not permit representation of relationships between relationships in such a case the
consider the entity type dependent related to employee entity which is used to keep track of the dependents of every employee the attributes of
entity types that do not have any key attribute and therefore cannot be identified independently are known as weak entity types a weak entity can be
the participation constraints shows whether the existence of an entity depends on its being associated to another entity by the relationship type
a relationship set is a set of relationships of the similar typefor example let consider the relationship among two entities sets course and student
let us first describe the term relationshipswhat are relationshipsa relationship can be defined as a set of associations or connection or a rule for
attributes that are directly kept in the data basefor example birth date attribute of a person derived attributes that are not stored directly
attributes having a one value for a particular entityfor instance age is a single valued attribute of a student entityattributes that
attributes attached to an entity can be of various typessimplethe attribute that cannot be further separated into smaller parts and shows the basic
every simple attribute of an entity type have a possible set of values that can be attached to it this is known as the domain of an attribute an
an entity type usually has an attribute whose values are distinct for every individual entity in the collection such an attribute is known as a key
a collection of a same kind of entities is known as an entity set or entity typeexamplefor the college database defines earlier objects of concern
entitieslet us first be aware of the questionwhat are entities an entity is an object of concern used to show the things in the actual world eg table
we will explain here an example database application having college database and use it in showing several e-r modelling conceptscollege database
let us first look into some of the important features of er model entity relationship model is a high-level conceptual data model it permits us to
exampleconsider the following relationsrollnonameaddresscourseid100kanupriya234 saraswati viharcs1101rajni bala120 vasant
when every condition appears with equality condition only c natural join denoted by rswhen two join attributes have the similar name in both
the join operation is applied on two relations when we want to choose related tuples from two given relation join is used the join operation need
consider the relation person if you want to display details of persons having age less than or equal to 30 than the select operation will be used as
let us now talk about the relational operationsselectthe select operation is used to select few specific record from the database based on different
if r1 and r2 are two relations or functions then the result of r3 r1 times r2 is the combination of tuples that are in r1 and r2 the product is
set differenceif r1 and r2 are two union compatible relations or relations then result of r3 r1- r2 is the relation that have only those tuples that
if r1 and r2 are two union compatible relations or functions then the result of r3 r1 capr2 is the relation that contains all tuples that are in
unionif r1 and r2 are two union compatible relations then r3 r1 cup r2 is the relation having tuples that are either in r1 or in r2 or in bothin