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I have never designed a database/data-model/schema from scratch, especially for a web-application.
In some recent job interviews, i was asked to 'design' a database for a shopping cart application. Now i am working on a mobile shopping application (retail, uses phonegap) with a backend that needs to store and process product and order info. The scale of this problem is so huge, i don't know where to start. I was hoping for some advise on -
How should I approach such a problem (shopping cart application DB) ? where should i start ?
Are there any common mistakes/pitfalls that i should avoid ?
What optimization/efficiency paradigms should i keep in mind when designing such a DB ?
How should i go about identifying entities in the problem space (products, orders, etc)? how should i derive the relationships between them ?
When a interviewer asks such a question, what exactly is he looking for ? is there something i should/should not say ?
I should also clarify that -
Yes, I am a noob, and my motives are to learn database design AND prepare for upcoming job interviews. I have read DBMS books where they describe individual concepts in detail, but i have no clue how to put those things together and start designing a database.
I have seen other threads on database design. The authors already tend to posses some knowledge on how to break the problem down. i would like to understand the methodology behind doing that.
Links to outside resources, comments, suggestions and anything that will put me on the right track is much appreciated. I hope this thread serves as a learning experience for myself and others.
There can be five tables in database:
CATEGORY this table stores information about products categories of your store and categories hierarchy.parent field of this table stores ID of the parent category.
PRODUCT all products of your store are stored in this table. This table has a foreign key categoryID which identifies ID of the category to which a product belongs.
ORDER this table stores information about all orders made by visitors of your store.
ORDERED_SHOPPING_CART table is tightly connected with PRODUCT and ORDER tables; stores information on customers' orders content.
SPECIAL_OFFER table contains a list of products, which are shown on home page as special offers
A brief answer is the way that i would tackle this problem. Firstly, there are loads of open source or free, web based shopping carts. This means that you can get one, set up the database and then have a good look around what they did.
Ask yourself questions such as, why have they done that? Why is it good? What downside could there be? How would i do it differently? why?
I would try to procure a database design tool that allows you to visualize the database. (like database designer in visual studio or i have one from MicroOlap that does pgsql databases)
Then you need to think about what you need in the database. What is the customer going to do? Buy products! Therefore you need a products table. Without going down the whole route you can see the point. Imagine what is needed, then basically make a table for it.
If you have more than one option for a field in a table, make another table with a relation in it. So if you have a product table and you have a status field. you could have more than one status. (eg out of stock, limited number, big item, expensive) instead of hard coding these fields, make a table and allow the user to add items to the table. then in the product table add a field status_id and link it to the status table
Many - many relationships are useful things to know. (i fell short to this myself.) say you have a component and product tables. The products can be made up of lots of components and the components could be allocated to many products. Create a mediator table. Something like prodcomp( and in this you would have fields like id, prod_id, comp_id, qtyneeded).
Learn to index correctly.
Don't create the database until you have a solid idea of how it will work. this saves time in recreating it later.
There may be more to this, however, i hope i have given you a good start.
Related
There is a Book table that is always unique with title, edition, and author.
And I want all bookstores to add their books, but different bookstores can have the same book with different pricing. So I come up with this table design.
So when one bookstore tries to add a book and the book is already been added by another bookstore the current bookstore should have to just fill in the pricing detail, not including the book detail.
The problem with this is, what if the book detail already been added has some missing or incorrect info? in this case, the current bookstore can flag and moderators or admins can fix it.
Is there any better way to achieve this? I don't comfortable with this design logic at all.
Your design makes sense. You want to keep the "static" information in 1 table, and link "dynamic" information like you did.
Your other question is related to data integrity. You can put "not null" conditions on fields to ensure all fields are filed, but garbage entries are always possible. This is a universal problem.
Potential solutions to mitigate this:
any and all data that can be selected instead of typed in should be linked via another table. Ex:
BookGenre
bookgenreid PK
genre CHAR
Book
bookid PK
genre FK, BookGenre.bookgenreid
...
So you store all possible genres in a separate table, so your users cannot invent new genres or mistype values. Same for authors, countries, ... This makes it easier to build queries as well and avoid things like [ SciFi, Science Fiction, Sciance fiction, ... ]
not everyone should be able to enter new books in the system. Ex. when I worked at a wholesale distributor, only a select group employees could create new products in the database, and they had established a convention on how to do it. They worked closely with purchasing and receiving. You will need to dedicate "data administrators".
So try to control as much as you can in the database and - or the application. Avoid free text fields as much as possible, as users will always think of new ways to mess it up. Ex. at work currently we have a HUGE project to standardise addresses between unlinked systems. It is a enormous undertaking, which involves AI. All this only because no 2 persons enter addresses exactly the same.
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Ok, maybe it's no the best title for the question, but this is the case.
I'm working on a project that already has an e-commerce. And part of the database looks like this. Everything works perfectly.
The problem comes with the references, if a user buys a product the shopping cart is closed, but if then the product is deleted or it's price changes the order becomes totally corrupted.
I've read this text -> Database Design for Real-World E-Commerce Systems
but I can't see the solution here.
What is the best way to do this. How big companies deal with this problem.
I mean what I need is to store all the details of an order with the data it had at the purchase moment.
There are different ways to solve this. One approach is to have a price history table rather than a price column that changes periodically. When you create an order you create it for a given price and given product. When you need to change the price of the product, instead of changing the value of the price column, you enter a new record in your price history table so future orders can then take the new price. Another approach is to decouple the product price information from the order. Rather than take the price from the product table, you have a column for unit price in the order table and the current value for the price is saved there.
As far as deleting products, it depends again on your situation. Generally it's not a good idea to delete rows that are needed for historical information. So if you no longer want to sell a product, rather than delete the record, you could have a column that has the availability of the product set to false. So previous orders would still relate to that product but new orders wouldn't be able to add it.
Every instance of every field in a table should have a 1:1 relationship with that instance of the table.
The problem is that the Price has a 1:1 relationship with the Product, which is good. But it should also have a 1:1 relationship with the Cart. And, since the price can change over time, it does not.
Two possible solutions: 1)Put a Timestamp on the Purchase, and keep all HistoricalPrices, then select the proper Price for the time of the purchase. This has the advantage of being able to tell exactly why the price changed, but can be extra work. 2)Add a PurchasePrice field to the Shopping_Cart_Products table. Assign the PurchasePrice value for that instance of that table at the time the purchase is made.
You have a few choices, if the products change while they're sitting in carts:
You update the cart and notify the user of the reason why.
You put all the information you need into the cart and work with that during checkout instead of the live product data.
Solution 1 requires you to do a sanity check at least once at the end of the checkout, to see if the cart is still valid. Solution 2 means that people may buy products that are somehow outdated.
Im building a friendship site where I try and match users who share similar interests.
I have 25 questions with defined answers(drop down answers) that the user must fill out.
Im using an entity–attribute–value model to store the users id the question id the answered id the user selects.
I then use the count function to see which users have the most matches to my profile.
Current Table Structure
Question Table
Answer Table
Question_Answer_User Table
The problem im running into is I have two question and im not sure where the best place to store them is.
The question is what is your country?
The question is what is your State?
Im not sure if I should store them with the other 25 questions or if i should store them in a three separate tables as seen below.
country table
state table
user_country_state Table
There are going to be alot of answer entries for these 2 questions. For example there are 25 countries the user can answer and a total of 900 states / provinces the user can answer from.
I want to be able to consider the users location as similarity to count but im not sure what the best approach to incorporate this is?
I think the selected country and state should live in the user table, along with the other necessary user information such as name and email address. I don't think it belongs in the Answers table, but it would work there.
For the list of options for the user to select from when setting up his account, storing them in your pre-defined Q and A tables are as good a place as any. It depends, I guess, on how your data and functionality is broken apart so that you aren't crossing responsibility boundaries for the Q&A table storing survey-type answers as well as user-setup answers.
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As part of a recent project I have started planning out, I am required to build the structure of a database which will contain several products. As an example, think of the way Amazon is structured. It has several categories and within those categories, several sub-categories.
My problem is that conceptually I am unsure on how to build the database tables. I have thought of creating a self-referencing table for the categories and sub-categories, but since I do plan to have a wide variety of products within the database, I don't know if I should just group them into one table called "Products" or put them all in separate tables.
For example, a toilet would be one product while a television could be another. Even though they have different categories/sub-categories, they are both products. By placing them in one "Products" table, they would share attributes that would make no sense for both of them. A toilet would not need an attribute for resolution or display size(unless it is a very special toilet?) and a television wouldn't need a seat size attribute.
I thought that one to get around this and still keep everything in one table would be to create a bunch of NOT NULL attributes that could be missing for certain items if they weren't necessary, but common sense is telling me that this is probably not the best way to go about things.
So at this point, I feel that my real problem is figuring out how to structure this database and its tables with several categories/sub-categories and different kinds of items. Would I create a table for televisions and a table for toilets? How would this all be structured? How are these sort of problems normally planned out?
Thanks
A generic products table is a good way to go. You're not going to want to create a new table in your schema every time you have a new type of product.
Similar with the categories, a self referencing table is better with a parent/child relationship so you don't have to create a new table each time you want a new level of sub-category.
Your products table should contain information that's common amongst all your products. E.g. name and possibly price (although if you have different prices for an individual product, then price is best stored in another table that references the product).
If you have a bunch of other information that relates to characteristics for each product, then maybe create an attributes table and another table that references each attribute's value for that product.
Here's a simple example schema:
This is more of a design decision than anything else.
This is how I would separate the tables:
categories (e.g. household)
sub_categories (e.g. bathroom is a foreign key of household)
products (e.g. Ceramic toilet)
As for the extra attributes, you can either store these directly within the products table or create another table called products_extra_attributes and store an optional NULL value within the products table which would be a foreign key pointing toward the additional attributes for the individual product.
Make sense? I'll make an edit later on if not as I'm answering this question from my phone.
Depends on how many products. If you only sold toilets and televisions I'd say go ahead and make totally separate tables for them, however if you have 100s of different product types all of which would have different attributes I might suggest creating a products table that stored common attributes (they all have a cost and, probably, a size) then a product type table that specifies a set of attributes for each product type, then a attributes table to define the attributes and lat a product values table.
So for example, take a Sony TV. It would be in products with the price and a link to the product type, which would be TV. That would one to many join to attributes that all TVs had and Sony TV would have entries in the product values for each of those attributes. This way, you wouldn't have to redefine shared attributes, so when you started selling other things that had resolution, you could just add them to the product type.
Make sense?
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I'm working with Ruby on Rails, but this question I think is broader than that and applies to database design generally.
When is it a good idea to split a single model up into multiple tables? For example, assume I have a User model, and the number of fields in the model is really starting to add up. For example, the User can enter his website, his birthday, his time zone, his etc etc.
Is there any advantage or disadvantage to splitting up the model, such that maybe the User table only has basic info like login and email, and then there is another table that every User has that is something like UserInfo, and another that is UserPermissions, and another that is UserPrivacySettings or something like that?
Edit: To add additional gloss on this, most of the fields are rarely accessed, except on pages specific to them. For example, things like birthday are only ever accessed if someone clicks through to a User's profile. Furthermore, some of the fields (which are rarely accessed) have the potential to be extremely large. Most of the fields have the potential to be either set to blank or nil.
Generally it is a good idea to put things which have a one-to-one relationship in the same table. Unless your userbase includes the Queen or Paddington Bear, a user has just one birthday, so that should be an attribute of the USERS table. Things which have a one-to-many relationship should be in separate tables. So, if a user can have multiple privacy settings by all means split them out.
Splitting one table into several tables can make queries more complicated or slower, if we want to retrieve all the user's information at once. On the other hand if we have a set of attributes which is only ever queried or updated in a discrete fashion then having a separate table to hold that data is a sound idea.
This would be a situation for analysis.
When you find that a lot of the fields in such a table are NULLs, and can be grouped together (eg. UserContactInfo), it is time to look at extracting the information to its own table.
You want to avoid having a table with tens/hundreds of fields with only sparsely entered data.
Rather try to group the data logically, and crete the main table containging the fields that are mostly all populated. Then you can create subsets of data, almost as you would represent them on the UI, (Contact Info, Personal Interest, Work Related Info, etc) into seperate tables.
Retrieving a row is more expensive if it has many columns, especially if you usually need just some of the fields. Also, hosting stuff such as the components of an address in a separate class is a case of DRY. On the other hand, if you do need all fields of an object, it takes longer to execute a compound query.
I would normally not bother to distribute classes over several tables just to make the code more readable (i.e. without actually reusable parts like addresses).