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.
Related
I have a newbie question about a database I am trying to create. I have a list of publications, with this general order:
UID, Author, URL, Title, Publication, start page, end page, volume, year
Then I realized that there are multiple Authors and I began trying to normalize the Database for multiple Authors. Then I realized that the Order of the authors is important, and that a journal article could also have numerous Authors, between 1, and dozens, or possibly even more.
Should I just create a table with multiple Authors (null columns)(like 12 or something)? Or is there a way to have a variable number of columns depending on the number of authors?
Database model
You basically need a many-to-many relationship between Authors and Publications, since one author can write many publications, and one publication can be written by more than one author.
This require you to have 3 tables.
Author - general info about every author (without publications_id)
Publication - general info about every publication (without author_id)
AuthorPublication - columns author_id and publication_id that are references to tables Author and Publication.
This way you're not binding a specific author to a publication, but you can have more of them, and the same thing the other way around.
Additional notes
If you would like to distinguish authors' role in particular publication you could also add some column like id_role that would be a reference to a dictionary table stating all possible roles for an author. This way you could differ between leading authors, co-authors etc. This way you could also store information about people handling translation of the book, but perhaps you should then change the naming of Author to something less specific.
Order of appearance
You can ensure a proper ordering of your authors by adding a column in AuthorPublication which you would increment separately for every Publication. This way you would be able to preserve the ordering as you need it.
You have many to many relationship between entity Publication and entity Author.
Publication can have many authors, author can have many publications.
So, you should create table for this relationship. For example table Authors_Publications with columns: UID, author_id, publication_id, order
You should create author table which is many-to-many relation with the publication table
Author have some information
and publication also have informaiton
so should have tables like author and publication
both have primary key like author_id and pblication_id
and both key having many-to-many relationship
Actually your scenario is even more complicated.
A publication can have more than one author. An author can write more than one published article or book. That is a Many-to-Many relationship.
We always(*) represent a many-to-many with a third table, sometimes called. Bridge table. This third table, authorship, is a child table with at least two columns, both foreign keys holding the primary key from each of its parent tables, pub_ and author_ tables. We transform the Many-to-Many into a pair of One-to-Many relationships.
By the way, this books-author scenario is the canonical example used when teaching relational database design.
You can have additional fields on this third table. In your case, we need a priority_ column of an integer type to sort the list of primary vs secondary authors.
Each author’s compensation fee or royalty would be additional columns on this bridge table. If you were tracking each author needing to sign a contract for their work on that publication, the authorship_ table would have a date, date-time, or boolean column contract_signed_. So you can see that the bridge table represents anything to do with one particular author’s involvement on one particular publication.
(*) Not merely an opinion or suggestion. Relational database design is proven by entire books filled with mathematical proofs. This includes the need to break up a many to many with a third table. Relational database design is the only case of true information engineering backed by mathematical description and proofs. Search for relation (a field of mathematics), and doctors E.F. Codd and Chris Date to learn more.
I've been looking through different questions on here and I can't find something that exactly matches my situation.
I am designing a database for multiple types of users. I have one main User table which includes ID, Username, Password, PasswordSalt, AccountType (enum), and LastLoginDate. I need to have multiple types of accounts: Student, Parent, SchoolAdmin, SystemAdmin, Coordinator, and Teacher. I was originally thinking of having a separate table for each of these types of accounts, but I realized that SchoolAdmin, Coordinator, SystemAdmin, and Teacher all share the exact same data. These account types all have different permissions though. The Student and Parent accounts have extra information that they have to store.
I then thought about adding the information that the 4 identical tables share to the User table and then deleting those tables, but I came across another problem. I need to reference different types of accounts in other tables. For example, I had a foreign key for TeacherID in the Club table to show who the club sponsor is. If I add the information to the User table and get rid of those other tables, then how do I reference a specific account type in another table?
I have never designed a database like this so any help is appreciated.
There are three main ways of implementing inheritance on database models. Please check the links below, and study which is the best one to solve your problem. Nothing better to start analyzing this types of situations to become a good architect.
Single Table Inheritance
Class Table Inheritance
Concrete Table Inheritance
Each of the different approaches have their pros and cons so choose wisely.
First of all, I am a beginner in database area. I'm trying to translate my ER diagram to a database schema. I have a table called research paper and a relationship called citation which references to research paper again. One research paper may have references to many other research papers. Let's say a paper with id=1 citates papers with id=2,3,4,5. How can I accomplish this?
You would accomplish this by creating a junction table (http://en.wikipedia.org/wiki/Many-to-many_(data_model))
In systems analysis, a many-to-many relationship is a type of cardinality that refers to the relationship between two entities (see also entity–relationship model) A and B in which A may contain a parent row[clarify] for which there are many children[clarify] in B and vice versa. For instance, think of A as Authors, and B as Books. An Author can write several Books, and a Book can be written by several Authors. Because most database management systems only support one-to-many relationships, it is necessary to implement such relationships physically via a third junction table (also called cross-reference table),
In this case, since both sides point back to the same table, you'll want to make sure you name the two columns properly so that you can tell which is the research paper, and which are the citations for that paper.
Here's an article with a better representation of the tables (but not as good of an explanation): http://en.wikipedia.org/wiki/Junction_table
I'm designing a website with courses and jobs.
I have a jobs table and courses table, and each job or course is offered by a 'body', which is either an institution(offering courses) or a company(offering jobs). I am deciding between these two options:
option1: use a 'Bodies' table, with a body_type column for both insitutions and companies.
option2: use separate 'institution' and 'company' tables.
My main problem is that there is also a post table where all adverts for courses and jobs are displayed from. Therefore if I go with the first option, I would just need to put a body_id as a record for each post, whereas if I choose the second option, I would need to have an extra join somewhere when displaying posts.
Which option is best? or is there an alternative design?
Don't think so much in terms of SQL syntax and "extra joins", think more in terms of models, entities, attributes, and relations.
At the highest level, your model's central entity is a Post. What are the attributes of a post?
Who posted it
When it was posted
Its contents
Some additional metadata for search purposes
(Others?)
Each of these attributes is either unique to that post and therefore should be in the post table directly, or is not and should be in a table which is related; one obvious example is "who posted it" - this should simply be a PostedBy field with an ID which relates another table for poster/body entities. (NB: Your poster entity does not necessarily have to be your body entity ...)
Your poster/body entity has its own attributes that are either unique to each poster/body, or again, should be in some normalized entity of their own.
Are job posts and course posts substantially different? Perhaps you should consider CoursePosts and JobPosts subset tables with job- and course-specific data, and then join these to your Posts table.
The key thing is to get your model in such a state that all of the entity attributes and relationships make sense where they are. Correctly modeling your actual entities will prevent both performance and logic issues down the line.
For your specific question, if your bodies are generally identical in terms of attributes (name, contact info, etc) then you want to put them in the same table. If they are substantially different, then they should probably be in different tables. And if they are substantially different, and your jobs and courses are substantially different, then definitely consider creating two entirely different data models for JobPosts versus CoursePosts and then simply linking them in some superset table of Posts. But as you can tell, from an object-oriented perspective, if your Posts have nothing in common but perhaps a unique key identifier and some administrative metadata, you might even ask why you're mixing these two entities in your application.
When resolving hierarchies there are usually 3 options:
Kill children: Your option 1
Kill parent: Your option 2
Keep both
I get the issue you're talking about when you kill the parent. Basically, you don't know to what table you have to create a foreign key. So unless you also create a post hierarchy where you have a post related to institution and a separate post table relating to company (horrible solution!) that is a no go. You could also solve this outside the design itself adding metadata in each post stating which table they should join against (not a good option either as your schema will not be self documentation and the data will determine how to join tables... which is error prone).
So I would discard killing the parent. Killing the children works good if you don't have too many different fields between the different tables. Also you should bear in mind that that approach is not good to solve issues wether the children can be both: institution and companies but it doesn't seem to be the case. Killing the children is also the most efficient one.
The third option that you haven't evaluated is the keeping both approach. This way you keep a dummy table containing the shared values between the bodies and each of the bodies have a FK to this "abstract" table (if you know what I mean). This is usually the least efficient way but most likely the most flexible. This way you can easily handle bodies that are of both types, and also that are only of type "body" but not a company nor an institution themselves (if that is even possible or might be possible in the future). You should note that in order to join a post to an institution you should always reference the parent table and then join the parent with the children.
This question might also be useful for you:
What is the best database schema to support values that are only appropriate to specific rows?
We have a MySQL database with employees, companies and addresses tables.
The setup works like this: employees and companies each have an id. Since they are in two different tables, an employee can have id = 1 and the company can have id = 1. Both can have multiple addresses.
Now the address table has two columns that links it either to a company or to a employee:
element_id
element_type_id
element_type_id is either 1 = person or 2 = company
The whole thing is a bit more complex and there are many more tables, but that kinda explains the concept.
The problem is now that we would like to start using the Entity Framework and for that we need to define relationships with foreign keys.
But that sounds pretty much impossible with the setup that we currently have, does it?
Since the address table would need to be combined with the person and the company somewhow....
Any ideas?
This concept is known as polymorphic associations. I once asked a question about it, because I had a similar data structure that I wanted to make referentially sound. In terms of this question your Address corresponds with the Comment table and Employee and Company with Person etc.
The answer was excellent. If you can change the schema I would certainly go for it and use that approach.
If you can't change the schema you can always subtype your Address class in your EF model and use element_type_id as the discriminator column. You would create subtypes EmployeeAddress and CompanyAddress, where Employee refers to the former and Company to the latter. But as "the whole thing is a bit more complex" I'm not sure if this will be feasible in your situation.
You already meantioned yourself that this concept is not fitting into a relational database. It is not wrong, MySQL and all the other just don't support it. And it is not a normalized layout, so it might be regarded bad practice by database engineers.
You should think about separating the address from the relation by creating two additional tables: employee_adresses and company_addresses, each holding a relation to the company and the address or the employee and the address. This way you might get one address that is used for a company and many employees, which might be a good thing (normalized structure).