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
Related
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
I have an Eentity Framework model with a Table Per Hierarchy (Brand) and also a Table Per Type (Vehicle) inheritance like this:
(Vehicle and Brand are abstract classes).
So far so good, I can access derived entities on linq queries using Vehicle.OfType<> or Brand.OfType<> method.
Now, Brand entity is one to many related with Vehicle on my conceptual model, So the question is, how should I make relationships on EF model so I can keep using navigation properties between Vehicle and Brand but at the same time keep the consistency of the TPH inheritance on Brand?, my first approach was to relate only derived clases, like:
But if I do this, I have no access to Brand directly from Vehicle, so I would have to do a double relation (between derived and base), like:
This works for me now, but I still have a duplicated relationship somehow, is this right?, do you have a better approach?, am I'm making some silly mistake on my modelling?
It seems to me that the reason you are running into cross-linking in your model is because you are artificially separating Brand and Vehicle as top-level sibling entities. If you start with Brand, which seems essentially equivalent to Make, that becomes the true top-level entity. There is no need to separate Make for each vehicle type (car, motorcycle, truck, etc.); just introduce the entity Model between Make and Vehicle and I think that solves most of your cross-linking problems.
Then the relationships aren't strictly parent-child, but are more accurate as composition. So you have Make, which has a one-to-many composite relationship to Model, which in turn has a one-to-many composite relationship to Vehicle. Vehicles are instances of a Model, so there isn't really a parent-child relationship there either. With this structure, there is no need to branch the EF for each type of Vehicle, because that is just part of what is described by the Model entity.
I hope my answer is helpful and that I haven't missed any of the essential points of what you are trying to model-
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.
Consider that i have two entities with following relationship:
Entity A <-->> Entity B (one-to-many and inverse)
Now consider that i have another entity Entity C that contains all the attributes of Entity B and some others, with following relationship:
Entity A <-->> Entity C (one-to-many and inverse)
Now i can improve the architecture by making Entity B the parent of Entity C.
Entity B
^
|
Entity C
Now, my question is, will the attribute(s) AS WELL AS the relationship(s) be inherited by Entity C? Meaning, do i still need to keep the following relationship (separately)?:
Entity A <-->> Entity C
Also, i couldn't find a good example for entity inheritance in Apple documentation for Core Data. Does anyone know of an online resource that explains this, with example (preferably)?
Yes, attributes and relationships and everything else will be inherited. Be careful though, child entities like that will share the same table in sqlite with the parent entity. So if you have C inheriting from B, then a table will be created in sqlite that has the properties for both B and C which the obvious voids in the table. This is not too much of an issue with a simple inheritance like this but if you decide to get "creative" you can end up with your entire model in one table.
Now, my question is, will the
attribute(s) AS WELL AS the
relationship(s) be inherited by Entity
C? Meaning, do i still need to keep
the following relationship
(separately)?:
Yes all attributes and relationships are inherited.
Only little documentation is available at ADC
I have a base class Participants inherited by Artist, Author and TextWriter.
I have only one table in the data store:
Participants {
ID,
FirstName,
LastName,
IsArtist,
IsAuthor,
IsTextWriter,
}
The idea is to have a class for all the roles that a participant can have.
I've managed to create the edmx file but when I try to get an Participant (as Artist) that is also an Author I receive the following error:
All objects in the EntitySet 'Participants' must have unique primary keys. However, an instance of type 'Artist' and an instance of type 'Author' both have the same primary key value, 'EntitySet=Participants;ID=1'.
Thank you
Yes, this is possible. What you're asking for is "table per hierarchy" inheritance. Your table needs to contain any "discriminator column" which identifies the type of each row.
However, no record for one person can have more than one concrete type when materialized (read from the DB), because an object can only have one type. I've written about this issue before:
One of the mental barriers that you have to get over when designing a good object relational mapping is the tendency to think primarily in object oriented terms, or relational terms, whichever suits your personality. A good object relational mapping, though, incorporates both a good object model and a good relational model. For example, let’s say you have a database with a table for People, and related tables for Employees and Customers. A single person might have a record in all three tables. Now, from a strictly relational point of view, you could construct a database VIEW for employees and another one for customers, both of which incorporate information from the People table. When using a one VIEW or the other, you can temporarily think of an individual person as "just" an Employee or "just" a Customer, even though you know that they are both. So someone coming from this worldview might be tempted to do an OO mapping where Employee and Customer are both (direct) subclasses of Person. But this doesn’t work with the data we have; since a single person has both employee and customer records (and since no Person instance can be of the concrete subtype Employee and Customer simultaneously), the OO relationship between Person and Employee needs to be composition rather than inheritance, and similarly for Person and Customer.
If "Bob" is a Participant who is both an Artist and an Author, then he cannot be of type, say, Artist and Author at the same time, unless one is a supertype of the other. Either Artist and Author should have a subtype relationship with the other or you should use aggregation rather than inheritance to relate Participant with Artist and Author. An instance of an object can have only one concrete type; this does not change because you store it to the DB.