I am just confused about One-One relationship and Many-Many relationship. Well if I have a users table and a course table, Do I have to add a third table to make it Many-Many relationship or I can just connect them with foreign keys and primary keys? I hope my question is clear to you.
One-to-Many example :-
Many to One example :-
Teacher teaches students is one to many relationship
And students learning from the teacher is many to one relationship
May be your dought is clear now!!
You are asking about many to many relationship. In this imaginary relation you have to follow following steps
1-create two tables(users,courses)
2-make third table which is called pivot table . that will contains the foreighn keys of both tables that shows the relation b/w two tables.
3-You also have to make relationships of one-to-many in both tables(users,courses).
I hope this helps if not visit this link you will understand.
https://onlinewebtutorblog.com/laravel-8-many-to-many-eloquent-relationship-tutorial/
Related
I have a diagram of entity-relationship. The question is how A relates to D. I understand how entities relate to each other but haven't found a good explanation of how something like this relates. Any help would be appreciated. Thanks.
A does not relates to D in any way. C has a relation with both A and D.
Eg let say c entity is person.
D entity is number of phone numbers
A entity is number of houses.
As you can see a person can have multiple houses and phones but there is no relation between a house and phone.
I have found a nice picture by briefly explaining the relationships:
So you once have a 1-to-0..* relationship and a 0..*-to-1 relationship.
EDIT:
I guess this is a many-to-many relationship. The diagram shows the representation in the database. And only for this relation a third entity is needed to store the keys.
If I have following ERD then what would be the key of the relationship?
MR ERD (cannot add the image directly)
I am confused with the relationship table? I am sure we have three tables for three entities and what about the relationship? Do I need a table? As in M:1, we don't need a table for a relationship. I can merge relationship with the entity at M side.
Any help will be appreciated.
I need to create a subdimension in icCube, for example:
I have two dimensions, the dimension Student and the dimension Localization. The dimension Student has a foreign key from Localization dimension and the fact table join with student dimension. Also I want create a hierarchy in student dimension with attributes from localization dimension.
Example of the model with localization as subdimension :
You've two options.
One dimension / two hierarchies
You would create a table that is the result of the crossjoin of Student and Localization (in the upcoming icCube version we've a join view). You'll use this
table to create a Dimension Student that has two hierarchies Student and Localization. Check Exists to see implication (it's usually not very important).
With this you only need to link Students to the facts as the relaction between facts and localization is solved through students ( it handles one-to-many too).
Two dimensions
You can create two dimension. One for Student and the other of Localization. Both are independent.
To bind the fact to the location we need to use an intermediate mapping structure that can be directly defined in the facts (it's called 'bridge', see Time). The bridge table would be STUDENT, ID_STUDENT, ID, ID_LOCALIZATION, this helps to build a mapping between ID_STUDENT and ID_LOCALIZATION (that might be many-to-many)
hope it helps
Im making an ERD now for medicine care.
I have an issue with 2 entities:
Patient and caregiver.
The caregiver is a person that helps for the patient and can do features on the application for the patient.
For now I choose that the patient is a separate user from the caregiver.
If its like that,I need to do a seperate entities for the caregiver and for the patient. When I do this , there is an overload on the ERD because the 2 entities can do the same thing on the system so they have the same connections to other entities.
In addition , they have just one attribute that is different between them and this is the diagnosis of the patient.
What to do?
I would suggest looking into adding the caregiver as a sub class if all the rows are the same apart from one. But having them both as separate entities wouldnt be an overload on the database as long as you add primary/foreign keys correctly. Have you got a current ERD i can look at?
Am making a cube in SQL Server Analysis Services 2005 and have a question about many to many relationships.
I have a many to many relationship between two entities that contains an additional descriptive column as part of the relationship.
I understand that I may need to a bridge table to model the relationship but I am not sure where to store the additional column - in the bridge table or elsewhere?
Many To Many relationsip in SSAS can be implemented via an intermediate fact table that contains both dimension key that subject to the relation.
For example; If you have a cube that has a book-sales-fact table and you want to aggregate total sales by author (which may have many books and a book may be written by many authors) you should also have a author-book intermediate fact table (just like in relational database world). In this bridge table, you should have both dimension keys (Author and Book) plus some measure related to the current book and author such as wages paid to the author to write the book (or chapters).
As a result, if your additional column is kind of a measure you should add that column to the intermediate fact table.