MadeEasy Test Series: Databases - Er Diagram

Question

What does this ---->  mean?

And How to solve ?

Comments

Thank You. Well explained.

Answer 1

See first of all we have to understand that relationship set cardinality will be defined by that entity where you are able to merge the relationship set.

As in the above ER diagram , E₁ is many to one relationship with each of the remaining entities E₂ , E₃ and E₄.As we know in case of one to many or many to one scenario , we merge the relationship set into that entity set which is in "many" set of the relation.The side where the arrow is shown is actually the "one side of the relationship set R.

Hence cardinality of R is also defined by E₁ since it is in "many side" of the relation and hence we can merge the relationship R with E₁ here.

Hence the maximum cardinality possible if all the E₁ are distinct since each instance of E₁ will map with atmost one of E₂ , E₃ and E₄(follows from the definition of many to one)

Alternatively , we can also understand this like :

The primary key of a relationship in case of many to one relationship is governed by the "many side" of the relationship set i.e. primary key of relationship in this case will be primary key of the entity which is in the "many" side.

So maximum no of relationship = maximum no of distinct entries possible for an entity in the "many" side = 50.

So maximum possible entries in R = no of elements in E₁   =  50

Hence 50 should be the correct answer.

Comments

while modelling an e-r diagram having n-ary relationship, the relation for R will have n-attributes (n- entitis to whichRi is attached) with all fthem combined to generate the primary key. Is this concept wrong?

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It depends actually on the cardinality of the entities of the participating entities.Say a relation is many to many , then all the candidate keys of participating entites is required for a relationship set.In fact in this case , we require a separate table for relationship.

Now suppose a relationship is many to one relationship , not separate table for relationship is required since we can merge them on the many side of the relation.In that case only the "many" side will contribute to the key and not the "one" side.

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yeah!! in general relatoships its fine, i am getting confused bcz of a n-ary relaationship. if in above e-r diagram, out of 4 entities two are having nth side of the relationship and remaining 2 havng 1 side, then?