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.
Related
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/
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.
I'm really new to setting relationships between tables.
I created 4 tables and they're in a one-on-one relationship, so I want to make these tables share the PK.
I'm using db browser for sqlite.
Thanks
How to show composite keys in Chen E-R Diagram? I have not used this notation and I really don't like it. Any references would suffice, as I have excellent understanding of data modeling concepts (what a brag!).
Note: I am aware of the symbol for associative entity, in such a symbol, one may have an auto-increment key as the PK, but then again, one may use the 2 FKs. In the later case, the PK would be composite. I don't know how to show it since Chen attribute symbol is an oval for each attribute.
Thanks.
You can underline the text of each attribute that is involved in the primary key:
Note that an associative entity set normally has a composite key. Introducing a surrogate key changes it into a regular entity set with its own identity. Associative entity sets are really just relationships which are the subjects of further relationships, and relationships are identified by the entity keys of the related entity sets.
Quick Summary: I'm building an ERD diagram and got lost in connecting two tables.
Introduction:
I'm building an ERD diagram for my project. Idea is pretty straight forward: I can type information about the company and it will be saved in the database. Later, I can see the list of submitted companies, as a list. As of now, I've information on the "paper", which I've implemented into my ERD diagram, so that later I can tell database, what exactly must be saved and where.
I have a primary table "Company_Info" which stores all the information about the company in the database. Using common sense (or not :)) I've created "Foreign Tables" that would store information about the company: "Gallery Images", "Opening_hours", etc. This way I believe, the database would more or less clear and readable by others.
ERD Diagram with the Description of all relationships
Problem Statement
The idea was to create a simple gallery for the company, so that they can upload their Product-pictures. If it's possible I would like to talk about the relationship between the Gallery and the Company tables. The way I see it, I think it should be like this:
One company can have 0,1,2 or many images. (Gallery_Images Table)
Images must be assigned to only 1 company. (Company_Info Table)
The relationship is one mandatory to many optional. (Many images & one company)
Question: I think the relationship between the Gallery_Table and Company_Info table will not work. Reason: Wrong relationship. I'm confused about the connection between those two tables.
I have made a connection via company PK & FK. This way, I think, the database would know what images belong to that specific company.
Confusion is with gallery_id in Gallery_Images table. Shouldn't it be connected with the company_info table too?
Image(BLOB) is a repeating group in Gallery_Images. This is not a problem if your purpose is to draw a diagram of an ER model.
If your purpose is to draw a diagram of a relational model, then the repeating group is a departure from First Normal Form. Relational schemas that depart from 1NF are generally subject to terrible problems, both with regard to performance and with regard to data management itself.
I don't see the Gallery table in your diagram. Did I miss something?