I'm managing to create my first complicated J2E Solution and in every tutorial I find some sort of intermediary tables usage, like here :
Tables : User, User_Roles, Roles
While logic would simply add a key to user Table referring to it's role on Roles table, why the usage of that intermediary table ?
I thought it's one or two developpers choice, but everywhere I look for a tutorial, I find this sort of sql schema.
Is it better ? Does it help in something particular ? Speed, security ? Cause from a logic point of view, using one table User and a foreign key to Roles is better.
Thank you
This is a common database relationship modeling called M-N (Many To Many). A User can have many Roles, and a Role can be assigned to many Users, so you need the intermediary table. Here's another example: a Teacher can teach many Classes, and each Class can be taught by many teachers (during different semesters, for example). In this case you need a Teacher-Class intermediary table.
A different kind of relationship is 1-N (one to N). A User can have many Telephones, but each Telephone is owned by a single User. In this case, a User's primary key (PK) is exported as a foreign key (FK) into the Telephones table. No need for an intermediary table.
Related
Hello,
Having a massive difficulty converting a disjointed EER diagram to mysql
I believe the User entity has been setup correctly, just wondering how I would link User to a buyer/seller? Would I add a buyer/seller ID on the user then link that as a FK?
Here is mysql below.. Any help would be greatly appreciated, thanks
Your model shows that the subclasses Buyer and Seller are disjoint specializations of the User entity (A seller cannot be a buyer, and a buyer cannot be a seller) denoted by the circle with 'D'. Depending on your situation, you could challenge this design (can someone sell while having bought in the past?).
Each of these entities have different attributes not common to one another, except the ones shared by a User.
There are different ways of modeling this, but a what you might need for instance, is to have a Buyer table and Seller table. Both of these tables would then contain a foreign key to the User table.
A less normalized alternative is to have a User table that contains attributes from both subclasses, but then some of the attributes would be NULL if they're not applicable. They would then be distinguishable by a Role attribute.
Advised reading: Database Normalization
thanks for all your help, in the end I made buyer and seller tables with the same attributes and then linked them by foreign key back to the USER ID as otherwise I wouldn't have been able to access the attributes..
I understand how basic database relationships work but I am confused about the following/follower social media relationship in RDBMs.
Users table:
userId (PK)
email
name
Follow table:
followerId (FK referencing userId),
followeeId (FK referencing userId)
This seems to be the correct database design but what I really want to know if this is a one to many or a many to many relationship.
I think it is a many to many because a user can follow many users and be followed by many users. But it may be a one to many because each follow a user makes consists of one userId to follow. This isn't the perfect many to many example such as Students and Classes so it is confusing me. Thanks.
It looks to me like your database design is consistent with your use case.
You have many-to-many relation between users, where a user may :
follow many users
AND be followed by many users
To represent this relationship, you have created a bridge table, the follow table. This is the relevant way to proceed.
The difference with a the typical Student/Class relation is that both objects being in relation are stored in the same table (users) : this is why you end up with two foreign keys in the follow table referencing user.userId. But regardless of this specifity, this is conceptually the same type of relationship.
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.
I have a table called 'notes', on this table I need to track who made that note, but the problem is that the creator of the note can be a user stored in one of three possible tables:
users
leads
managers
I have though of simply create three fields on 'notes' to represent the three possible relations: note.user, note.lead, note.manager
With this approach I would be forced to create three table joins when requesting the notes to gather the creators information, and I don't think that is the way to go, so I would like to hear your ideas or comments and what would be the best approach on this.
For me personally this smells like a design problem on a totally different part of the schema: Are manageers not users? Do leads carry person information?
With any approach that creates a relation between one column and one of three others, you will need three joins for the select. If you can't rectify the underlying problem, I recommend you use
note_type ENUM('users','leads','managers')
as an additional field and
SELECT
...
IFNULL(users.name(IFNULL(managers.name,leads.name))) AS name
..
FROM notes
LEFT JOIN users ON notes.note_type='users' AND users.id=notes.note_source
LEFT JOIN managers ON notes.note_type='managers' AND managers.id=notes.note_source
LEFT JOIN leads ON notes.note_type='leads' AND leads.id=notes.note_source
...
for the query
I think you need to abstract out the concept of a user id, so that it does not depend on their role. The author of a note could then be specified by the user id.
Users could be assigned roles, and maybe more than one.
The correct way to structure this would be to pull all common data out of users, leads, and managers. Unify this data into a "contact" table. Then if you want to get all notes for a given manager:
managers->contacts->notes
for a lead:
leads->contacts->notes
Notice your original post: "the problem is that the creator of the note can be a user stored in one of three possible tables"
From the structure of your sentence you even admit that all these entities have something in common; they are all users. Why not make the DB reflect this?
you have to model a parent table for the three tables you already have. Define a table that depicts generally user, leads and manager tables. Something like "Person". So you have all of the ids of the three tables and any common attributes on the Person table. And when you must define the relationship you put the foreign id "Person_ID" on the note table. And when you model user, leads and manager tables you also put the primary key as a foreign key to the Person table.
So you would have something like this:
Table users:
Users(
person_id primary key
...(attributes of Users)
foreign key person_id references Person.person_id
)
This model i depict is common to any relational model you have to model using parents and childs
i wanna have a Users details stored in the database.. with columns like firstname, last name, username, password, email, cellphone number, activation codes, gender, birthday, occupation, and a few other more. is it good to store all of these on the same table or should i split it between two users and profile ?
If those are attributes of a User (and they are 1-1) then they belong in the user table.
You would only normally split if there were many columns; then you might create another table in a 1-1 mapping.
Another table is obviously required if there are many profile rows per user.
One table should be good enough.
Two tables or more generally vertical portioning comes in when you want to scale out. So you split your tables in multiple tables where usually the partiotioning criteria is the usage i.e., the most common attributes which are used together are housed in one table and others in another table.
One table should be okay. I'd be storing a hash in the password column.
I suggest you read this article on Wikipedia. about database normalization.
It describes the different possibilities and the pros and cons of each. It really depends on what else you want to store and the relationship between the user and its properties.
Ideally one table should be used. If the number of columns becomes harder to manage only then you should move them to another table. In that case, ideally, the two tables should have a one-one relationship which you can easily establish by setting the foreign key in the related table as the primary key:
User
-------------------------------
UserID INT NOT NULL PRIMARY KEY
UserProfile
-------------------------------------------------------
UserID INT NOT NULL PRIMARY KEY REFERENCES User(UserID)
Depend on what kind of application it is, it might be different.
for an enterprise application that my users are the employees as well, I would suggest two tables.
tbl_UserPersonallInformation
(contains the personal information
like name, address, email,...)
tbl_UserSystemInformation (contains
other information like ( Title,
JoinedTheCompanyOn,
LeftTheCompanyOn)
In systems such as "Document Managements" , "Project Information Managements",... this might be necessary.
for example in a company the employees might leave and rejoin after few years and even they will have different job title. The employee had have some activities and records with his old title and he will have some more with the new one. So it should be recorded in the system that with which title (authority) he had done some stuff.