I know this is not code realated but wasnt sure where to post it. If this is the wrong spot please let me know where to post as opposed to just a down vote. Basically I am setting up my first DB on my own. I am a little conffused on how to set some tables up. I understand the basic relational model by using a unique ID to pair up tables but what about an item in a table that has many relations.
Examble:
User Table
ID | Name | Email
Product Table
ID | Name | Price | User_ID
Now I know I can pair the 2 using the User_ID in the product table but what if the product can belong to multiple users? How do I associate the the product by using the User_ID to multiple users?
Related
I have a table where I store products, and in the same table I store the ID of the category I want the product to be in.
My product table looks like this:
id | product_name | category_id | price
And then I have my category table:
id | category
My problem is to know how I can insert multiple categories into my category_id, and if it is possible.
If not, whats the best way that I could do it?
It is possible but you really don't want to go there. Storing multiple values in a single datarow column is a terrible idea in 99.99999% of the cases.
For more information, read Is storing a delimited list in a database column really that bad?, where you will see a lot of reasons why the answer to this question is Absolutely yes!
What you want to do is to add another table to store the relationship between the products and the categories. This is referred to as a many to many relationship.
This new table should hold the product id in one column and the category id in the other one, and have a composite primary key that is the combination of both these columns.
This way, you can have many products in the same category, and many categories for the same product.
You need a many to many relationship, which mean you need a third table let's call it ProductCategories, in this case you will have
Products
id | product_name | category_id | price
Category
id | category
ProductCategories
PruductID | CategoryID (combined PK)
Ok, I am new to SQL, so this must sound stupid. Sorry for that.
I have two tables + one junction table (many to many schema) = total of three tables.
First table
students: student_id (PRIMARY) | name | email | password |...
Second table
topics: topic_id (PRIMARY) | topic_name | subject |...
Third table (junction)
jnct_students_topics: id (PRIMARY) | student_id_FK | topic_id_FK | done (boolean) | notes
So how the web app would work?
Second table (topics) is pre-filled with all the topics students need to study.
First table (students) will be filled whenever a new student register for the service.
Junction table (jnct_students_topics) will also be filled when student registers, but (HERE COMES THE QUESTION) I need to know how to insert all topics ids + this student id in topic_id_FK and student_id_FK columns.
Because when a student registers, system will create a list for him/her with all topics to study and a checkbox for each one. After student finishs with topic studying, he/she will check the box and the boolean done column will become true for this topic.
I learned from [this question] (Copy from one column to another (different tables same database) mysql) that, to populate a table column with values from another table column, I could go:
INSERT INTO jnct_students_topics (topic_id_FK)
SELECT topic_id
FROM topics
but I need to build the statement so at the same time the student_id (related to the student that is registering) is inserted (column student_id_FK) in all rows that are being created.
Tired of reading? Well, I don't blame you. Nor english or SQL are native languages of mine... :)
By the way, If you think that this schema is not the best, please, I would appreciate some advice.
Thanks!
I have two entities Invoice and User.
Invoice have following relations with user
invoice have a owner(one to one relation with user)
one invoice can share with multiple users, it means invoice can have multiple shared users(one to many relation with user)
How to map these relations in mysql database? how many tables? and table structure?
invoice could have two fields, billed_to and payed_by. your billed_to field would be your one to one relationship, and the payed_by field would be your one to many.
For One-to-one relationship, it is advised not to keep separate table. Keep it with the invoice table itself.
For one-to-many relationship, keep another mapping table.
Your table structure should be
Users
----------
id | xxx | .....
-
Invoices
------------
id | user_id | .....
in the above case user_id is the owner.
shared_invoices
-------------------
id | user_id | invoice_id
I'm working on a project in which a user can save their own coupon codes on the website, so I want to know what is the best way to do that.
Lets say, I have 1 table with the users, like this:
userId | firstName | lastName | codeId
And then I have a table of the coupon codes, like this:
codeId | codeNumber
So what I can do is to connect the codeId to userId so when someone saves the coupons the codeId from the coupon table goes into the codeId of the users table.
But now what do I do if when user has multiple coupons? How should they be connected to the user?
I have 2 options what to do:
Saving the codeId from coupons table into the codeId of users table like 1,2,3,4,5,
Make a new row into the coupons table and connect the user to the code by adding another field in the coupon table userId and putting into it the userId of the user which has added the coupon.
So which of the two options is better?
A many-to-many relationship between two tables should be solved by adding a third table connecting the two:
user_coupons
- userId
- couponId
Your first option violates the 1-NF rule, where it says that no column will have a group of values.
This is a typical one-to-many relationship. Your second option of creating a separate table User_Coupons (not the the underscore) and create a row with UserID, CouponCode, for each association. The two columns together should be unique so that they both become the primary key of the associative table.
If i was you my database will be:
userId | firstName | lastName
codeNumber | userId
You can insert many codeNumber for any user if you want.
I need to sell items on my fictitious website and as such have come up with a couple of tables and was wondering if anyone could let me know if this is plausible and if not where i might be able to change things?
I am thinking along the lines of;
Products table : ID, Name, Cost, mediaType(FK)
Media: Id, Name(book, cd, dvd etc)
What is confusing me is that a user might have / own many products, but how would you store an array of product id's in a single column?
Thanks
You could something like store a JSON array in a text or varchar field and let the application handle parsing it.
MySQL doesn't have a native array type, unlike say PostgreSQL, but in general I find if you're trying to store an array you're probably doing something wrong. Of course every rule has its exceptions.
What your probably want is a user table and then a table that correlates products to users. If a product is only going to relate to one user then you can add a user ID column to your Products table. If not, then you'll want another lookup table which handles the many to many relationship. It would look something like this:
------------------------
| user_id | product_id |
------------------------
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 2 |
| 3 | 1 |
| 3 | 5 |
------------------------
I think one way of storing all the products which user has in one column is to store it as a string where product ids are separated by some delimiters like comma. Though this is not the way you want to solve. The best way to solve this problem would be to have a seperate user table and than have a user product table where you associate userid with product id. You could than simple use a simple query to get list of all the products owned by a particular userid
As a starting point, try to think of the system in terms of the major parts - you would have a 'warehouse', so you need a table to list the products you have, and you are going to possibly have users who register their details with you for regular visits - so an account per user. You would generally hold all details of a single product in the same row of the same table (unless you have a really complex product to detail, but not likely). If you're going to keep track of products bought per user account, there's always the option of keeping the order history as a delimited list in a large text field. For example: date,id,id,id,id;date,id,id. Or you could simply refer to order numbers and have a separate table for orders placed [by any customer].
What is confusing me is that a user might have / own many products, but how would you store an array of product id's in a single column?
This is called a "many-to-many" relationship. In essence you would have a table for users, a table for products, and a table to map them like this:
[table] Users
- id
- name
[table] Products
- id
- name
- price
[table] Users_Products
- user_id
- product_id
Then when you want to know what products a user has, you could perform a query like:
SELECT product_id FROM Users_Products WHERE user_id=23;
Of course, user id 23 is fictituous for examples sake. The resulting recordset would contain the id's of all the products the user owns.
You wouldn't store an array of things into a single column. In fact you usually wouldn't store them in separate columns either.
You need to step away from design for a bit and go investigate third normal form. That should be you starting point and, in the vast majority of cases, your ending point for designing database schemas.
The correct way of handling variable size "arrays" is with two tables with a many to one relationship, something like:
Users
User ID (primary key)
Name
Other user info
Objects:
Object Id (primary key)
User id (foreign key, references Users(User id)
Other object info
That's the simplest form where one object is tied to a specific user, but a specific user may have any number of objects.
Where an object can be "owned" by multiple users (I say an object meaning (for example) the book "Death of a Salesman", but obviously each user has their own copy of an object), you use a joining table:
Users
User ID (primary key)
Name
Other user info
Objects:
Object Id (primary key)
User id (foreign key, references Users(User id))
Other object info
UserObjects:
User id (foreign key, references Users(User id))
Object id (foreign key, references Objects(Object id))
Count
primary key (User id, Object id)
Similarly, you can handle one or more by adding an object id to the Users table.
But, until you've nutted out the simplest form and understand 3NF, they won't generally matter to you.