I want to know in what situations we create many to many relation. What is the need of doing such?
A quick search goes a long way. Though the following is for MS Access, the concept is the same for any relational database.
Via: office.microsoft.com - Create a many-to-many relationship:
You have a many-to-many relationship when a single record in one table
can relate to many records in another, and a single record in that
second table can also relate to many records in the first. For
example, say your company has several types of computers and several
technicians, with each technician certified to work on some, but not
all, of the computers. Each technician can be related to more than one
computer, and in turn, each computer can be related to more than one
technician.
To track who can work on a given machine, you create a many-to-many
relationship by adding the primary keys from both sides of the
relationship to a third table, called a junction or link table. In
other words, a many-to-many relationship is really just a pair of
one-to-many relationships.
Related
I have the following Entity Relation diagram (MySQL), which I made for a small project(for the university). I would like to know if the relation between PK and FK is properly made.
I am aware that token should not be used in the users table.
Don't know why my brain cannot comprehend the relationships properly. As far as I've managed to come up is:
Many users can have one role. (User, Admin, University)
One role can be belong to many users.
One user can have one event.
One event can belong to one user.
One user can have many attendings.
Many attendings can belong to one user.
Many events can have many attendings. (Although One event can have many attendings?)
Many attendings can belong to many events. (Although many attendings can belong to one event)
I did another diagram using Reverse Engineering from Workbench, and it gave me almost the same. The only difference is that between events and attenders is a one-to-many relationship.
Sometimes I look at them and I am not aware of which question is the best when 2 questions are possible. Is it one too many, or many too many.... This is a tricky question for my brain. It is like there is a missing link that I cannot grasp, and I am pretty sure it is not that hard.
And this is just a simple relationship, don't wanna know the smack my brain would give me if I were to make a 10-20 tables diagram ... Any other criticism is welcomed :) Thank you!
Many users can have one role. (User, Admin, University)
One role can be belong to many users.
One user can have one event.
One event can belong to one user.
One user can have many attendings.
Many attendings can belong to one user.
All of these statements are correct. From my understanding, you're having difficulty describing the relationship between users and events. Essentially what you have is a many-to-many relationship between users and events. In order for a many-to-many relationship to exist in standard relational systems, there must be a join table. The attenders_to table is serving the purpose of a join column in this relationship.
There also appears to be a separate relationship between events and users, in that each event has a direct column for a userId (I imagine this represents the person hosting/putting on the event). This can be seen as almost independent of the other relationship.
So in short, there are actually two relationships between Users and Events: A Many-to-Many relationship describing the people attending the event, as well as a One-to-Many (that is, one User to many Events) relationship describing the host of the event.
Let me know if this helps clear things up or if there's anything else that I'm missing!
I was studying about relationships in RDBMS.I have understood the basic concept behind mapping relation ship,but I am not able to spot them.
The three possibilities :
one to many(Most common) requires a PK - FK relationsip.Two tables involved
many to many(less common) requires a junction table.Three tables Involved
one to one(very rare). One table involved.
When I begin a project,I am not able to separate the first two conditions and I am not clear in my head.
Examples when I study help for a brief moment,but not when I need to put these principles in to practice.
This is the place where most begineers falter.
How can I spot these relationships.Is there a simpler way?
Don't look at relationships from a technical perspective. Use analogies and real-life examples when trying to envision relationships in your head.
For example, let's say we have a library database.
A library must have books.
M:M
Each Book may have been written by multiple Authors and each Author may have written multiple Books. Thus it is a many-to-many relationship which will reflect into 3 tables in the database.
1:M
Each Book must also have a Publisher, but a Book may only have one Publisher and a Publisher can publish many Books. Thus it is a one-to-many relationship and it reflects with the PublisherId being referenced in the Books table.
A simple analogy like this one explains relationships to their core. When you try to look at them through a technical lens you're only making it harder on yourself. What's actually difficult is applying real world data scenarios when constructing your database.
I think the reason you are not getting the answers that you need is because of the way you are framing the question. Instead of asking “How do I spot the correct type of relationship between entities”, think about “How do my functional needs dictate what relationship to implement”. Database design doesn’t drive the function; it’s the functional needs that drive the relationships you need to implement.
When designing a database structure, you need to identify all the entities. Entities are all the facts that you want to store: lists of things like book titles, invoices, countries, dog species, etc. Then to identify your relationships, you have to consider the types of questions you will want to ask your database. It takes a bit of forward thinking sometimes… just because nobody is asking the question now doesn’t mean that it might not ever be asked. So you can’t ask the universe “what is the relationship between these lists of facts?” because there is no definitive answer. You define the universe… I only want to know answers to these types of questions; therefore I need to use this type of relationship.
Let’s examine an example relation between two common entities: a table of customers and a table of store locations. There is no “correct” way to relate these entities without first defining what you need to know about them. Let’s say you work for a retailer and you want to give a customer a default store designation so they can see products on the website that their local store has in stock. This only requires a one-to-many relationship between a store and the customer. Designing the relationship this way ensures that one store can have many customers as their default and each customer can only have one default store. To implement this relationship is as easy as adding a DefaultStore field to your Customer table as a foreign key that links to the primary key of the Store table.
The same two entities above might have alternate requirements for the relationship definition in a different context. Let’s say that I need to be able to give the customer the opportunity to select a list of favorite stores so that they can query about in stock information about all of them at once. This requires a many-to-many relationship because you want one customer to be able to relate to many stores and each store can also relate to many customers. To implement a many-to-many relationship requires a little more overhead because you will have to create a separate table to define the relationship links, but you get this additional functionality. You might call your relationship table something like CustomerStoreFavorites and would have as its primary key as the combined primary keys from each of the entities: (CustomerID, StoreID). You could also add attributes to the relationship, like possibly a LastOrderDate field to specify the last date that the customer ordered something from a particular store.
You could technically define both types of relationships for the same two entities. As an example: maybe you need to give the customer the option to select a default store, but you also need to be able to record the last date that a customer ordered something from a particular store. You could implement the DefaultStore field on the Customer table with the foreign key to the Store table and also create a relationship table to track all the stores that a customer has ordered from.
If you had some weird situation where every customer had their own store, then you wouldn’t even need to create two tables for your entities because you can fit all the attributes for both the customer and the store into one table.
In short, the way you determine which type of relationship to implement is to ask yourself what questions you will need to ask the database. The way you design it will restrict the relational data you can collect as well as the queries you can ask. If I design a one-to-many relationship from the store to the customer, I won’t be able to ask questions about all the stores that each customer has ordered from unless I can get to that information though other relationships. For example, I could create an entity called "purchases" which has a one-to-many relationship to the customer and store. If each purchase is defined to relate to one customer and one store, now I can query “what stores has this customer ordered from?” In fact with this structure I am able to capture and report on a much richer source of information about all of the customer's purchases at any store. So you also need to consider the context of all the other relationships in your database to decide which relationship to implement between two particular entities.
There is no magic formula, so it just takes practice, experience, and a little creativity. ER Diagrams are a great way to get your design out of your head and onto paper so that you can analyze your design and ensure that you can get the right types of questions answered. There are also a lot of books and resources to learn about database architecture. One good book I learned a lot from was “Database System Concepts” by Abraham Silberschatz and Henry Korth.
Say you have two tables A and B. Consider an entry from A and think of how many entries from B it could possibly be related with at most: only one, or more? Then consider an entry from B and think of how many entries in A it could be related with.
Some examples:
Table A: Mothers, Table B: Children. Each child has only one mother but a mother may have one or more children. Mothers and Children have a one-to-many relationship.
Table A: Doctors, Table B: Patients. Each patient may be visiting one or more doctors and each doctor treats one or more patients. So they have a many-to-many relationship.
An example of one to one:
LicencePlate to Vehicle. One licence plate belongs to one vehicle and one vehicle has one licence plate.
I have three tables: Resumes, Orgs, and Resume2Org. Basically, Resume2Org is my many-to-many relationship table linking Resumes.resume_id to Orgs.org_id (so it only has those two keys in that table).
My question is, is it okay to use that many-to-many relationship table to store other data? My use case: the database is part of a system to sift through incoming resumes. But I've been asked to implement a "marked as read" feature so we can easily get the list of resumes we haven't looked at yet. But since a resume can belong to many different orgs, we only want to mark a resume as read for the org the user/viewer belongs to. I thought, hey, having that flag in Resume2Org would be perfect. Is this a smart approach, or should I create a new table specifically for "marked as read"? All the examples I've seen about many-to-many relationship tables is that those tables are used just for that... linking two tables.
Yes it is okey to have additional fields in a many-to-many table. I think it is the right way to do in your case as you don't need to join additional tables and you save spaces.
I was in a very similar situation last week and I added additional field for that.
I have the following tables created:
Animes(id,title,date), Comics(id,title,date), TVSeries(id,title,season,episode,date)
Some of them have already foreign keys (for one-to-many or many-to-many relations) of directors, genres, articles and so on.
Now i would like to create two more tables Reviews(id,rating,date) and Posts(id,thumbid,articleid,reviewid).
A review is about one Anime and/or Comic TVSerie and vise-versa but properties of a review may be in more than one table. Its the same about a posts.
Is this a typical example of one-to-one relation in separate table or is it more efficient to add more properties to the existing tables?
So more tables and relations or less tables more columns?
Thank you and i hope my question isnt that stupid but im a bit confused.
In my view, It is better to avoid foreign key relationship for one-to-one relationship. It is best suitable for one - many relationships.
I'm not exactly sure what your requirements are, but the choices are as follows:
Have Reviews have 2 columns, either being a foreign key to the applicable table, can be NULL. This is really for when a single review can be about both.
Have a ReviewsComics and ReviewsAnime table. You'd then have all the fields from Reviews in each table (and no Reviews table).
An alternative (2) is to use them in conjunction with a Reviews table, then those 2 tables only has 2 fields which are foreign keys to Reviews and Comics/Anime respectively (thus no direct link between Reviews and Comics/Anime).
Have a base table to which Anime and Comics are linked to 1-to-1 and have reviews link to that table instead.
(Edit) If all the fields are all going to be the same (or similar) for Anime/Comics, you can merge them into a single table and add a type field, indicating Anime/Comics, then the problem goes away. This is similar to the base table option.
EDIT: The 2 reviews tables will probably give the best performance (unless you want to select all reviews for either, often), but with proper indices the performance shouldn't be an issue with any of the above.
I am trying to lay out the concept for a Relational DB and I ran into some conceptual Problems:
If I have multiple discrete Entities that are "nested" in each other/have a hierarchy e.g.:
Bosses can have multiple Employees. These employees have different Projects they do and One Project has again multiple sections.
So
B1-Bn:
E1-En
P1-Pn
Section1 -SectionN
How would that be best mapped in a database?
Or in other words, how is this hierarchy best mapped in a relational db?
Now I have Costumers that interact with these Employees.
They are met by the bosses
Then they decide which employee will work for them.
Then they are assigned Projects, with one or more sections.
How would that be best mapped.
the Relations 1-n, m-n, 1-1: Can they be used for e.g.:
This is a Foreignkey because of the 1-n relationship.
This is a ManytoManyField because of the m-n relationship.
And is there a excellent online tool to better understand/visualize that.
Thanks so much for your time!
You may want to follow a course on relational database design; this subject takes more than a few days to explain or master. But you are on the right track.
The first thing you may be seeing is a hierarchy, but before you know it there will be relationships that aren't hierarchical, so a network is formed.
This is why relational databases do not work with hierarchies.
You identify different types of entities and have one table for each type.
For each entity type you identify the properties of such entities - each property will be a column of the table.
If a property does not have an atomic value, but a structured value, these structured values must be regarded as an entity, and must be given its own table, and the property will be a foreign key referring that table.
In this way, you will form a network of tables linked by foreign keys. This is called an entity-relationship diagram. Many designers advocate creating such a diagram first, without mapping the entity types to tables directly. They allow many-to-many relationships between entity types in the diagram. A foreign key between tables on the other hand is always many-to-1 or 1-to-1. So these designers have an "implementation" step in which they introduce an additional table for each many-to-many relationship. Personally I don't use many-to-many relationships in my diagrams to begin with.