I understand that the types of relationship used in an ER diagram are
One to One,
One to Many,
Many to One, and
Many to Many
If I am to form a relationship between 'Team' entity and 'Match' entity related by 'plays', where each match is played by two teams. What is the kind of relationship here?
it is many to many. Because we dont have 2 to 2 relationships in ER diagrams.. therefore when it is more than 1 it is many.
Related
I am new to the RDBMS.
I am Learning ER model in RDBMS.
In ER model, the entity is an real world object and it has an attributes.
The relationship is an mapping between entity set.
The relationship also have a the attributes.
Please explain the difference between entity and relationship.
You seem to have the definition differences available. But I assume you still do not understand the differences. Here is a very simplified example of two entities and the relationship that may exist between them:
A Bank and a Person are each an entity. The relationship that exists between a Bank and a Person is that a Person is a Customer to a Bank. Therefore Customer is the relationship. An attribute for a Person for example would be Date_of_Birth. An attribute for a Bank would be Bank_Name . An attribute for a Customer would be Customer_Bank_Acc_Number.
Update:
For those that like to pick at details here is a better relationship example:
A Person can have a relationship with a Bank of either being a Debtor or Creditor.
Update
There is also what is called an Associative Entity. Click the link for details on how that is different to an Associative Relationship.
I hope this makes sense. Cheers
I'm a bit confused about this naming convention.
What is the difference between them and are target/source interchangeable with owning/not owning?
One thing in particular is hard to understand:
"The main difference between a OneToOne and a ManyToOne relationship in JPA is that a ManyToOne always contains a foreign key from the source object's table to the target object's table, where as a OneToOne relationship the foreign key may either be in the source object's table or the target object's table"
JPA wikibooks
I can't imagine such situation in uni one-to-one
Differences between them are a little confusing. You should practice a lot to understand very well.
At first, you should understand some terminology:
Role : In every relationship there are two entities that are related to one another, and each entity is said to play a role in the relationship.
Direction : Relationships can be unidirectional or bidirectional. For e.g.. a Person has an address is normally unidirectional whereas Employee working on a project is normally bidirectional. We will look at how to identify and define directionality while coming up with a Data Model.
In order to have relationships at all, there has to be a way to create, remove, and maintain them. The basic way this is done is by an entity having a relationship attribute that refers to its related entity in a way that identifies it as playing the other role of the relationship. It is often the case that the other entity, in turn, has an attribute that points back to the original entity. When each entity points to the other, the relationship is bidirectional. If only one entity has a pointer to the other, the relationship is said to be unidirectional. A relationship from an Employee to the Project that they work on would be bidirectional. The Employee should know its Project, and the Project should point to the Employee working on it. A UML model of this relationship is shown here. The arrows going in both directions indicate the bidirectionality of the relationship (Form this book >> Pro JPA 2)
Then dive into this link (archived from the original)
I'd like to comment only the links, but I need 50 reputation
Let's say there is an entity called Product, which is connected to Shop (a shop has several products). However, there is also an entity called SpecialWebOffers, which also has a number of Products (which are not in any Shop).
How do I connect both Shop and SpecialOffers to Product. Is it OK to have two relationships in Product - one to Shop and one to SpecialWebOffers?
It's very common for an entity to have multiple relationships. If I am understanding your question correctly then the answer is yes.
Say you have a couple of Core Data entities .. Student and Exam. These two are initially filled with data from two xml files downloaded from the web.
Now, Students and Exams are separate things... initially there are no connections between them. But after filling out these two entities, I might want to connect certain students to certain exams. Or I might want make all students take a particular exam. But I still want to be able to treat Exams as independent things, which might have no students connected.
I'm unsure how to do this with Core Data. In the data model, you either have a relationship or yo don't. Should I have two different entities for Exam... one for independent exams, and one connected to the student which can be built up from the other Exam enitity?
No, you should not make two entity types.
Just because you have a relationship between two kinds of entities doesn't mean you can't create an object where that relationship is nil.
So, assuming you have a many-to-many relationship between Student and Exam, you might create a new exam by doing something like:
Exam *newExam = [NSEntityDescription
insertNewObjectForEntityForName:#"Exam"
inManagedObjectContext:context];
newExam.course = #"CS 101";
newExam.description = #"Midterm";
You might then later establish a relationship between a student and that exam like:
[newExam.students addObject:aStudent];
(where students is the name of the relationship between Exam and Student
I think you should have a relationship between the two entities (exam and student) but mark it as optional.
What kind of relationships are called n-ary relationships??
Not sure what you mean by the term "n-ary", but normally relationships that both sides can have many of the other are termed many to many relationships and those that only one side does are termed one to many.