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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).
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I'm redesigning our service app, and getting rid of some really awful schema problems while I'm at it. Trying to build the replacement with best practices as much as possible.
I'm having a company table rather than just customer, as it's often useful to identify companies that are not customers (suppliers, contractors, etc etc). I'm trying to decide whether it's better to simply include a boolean field represented in the relevant part of the app by a checkbox that identifies relevant companies as customers (which would become uneditable once the customer has services attached to them), or if I should, instead, have a separate table that's basically just a single field referencing the Company ID that is in turn referenced by any child records.
This similar question asks about records that can be one of several subtypes. While the question is materially different (every policy seems to be only one of the potential subtypes, whereas Companies can be any or all of Customer/Supplier/Contractor etc) its similarity combined with the fact that it has multiple conflicting answers raises the possibility that there is no industry-wide consensus, so:
Is there an established best practice here? I'm not immediately seeing any reasons that other fields should be included in the prospective Customer table, but I'm open to the idea that there might... is that a good enough reason to go with B? Or is this a clear YMMV situation, where both options have benefits, either being equally valid?
I should, instead, have a separate table that's basically just a single field referencing the Company ID that is in turn referenced by any child records.
There are probably several attributes that apply to a customer that don't apply to a non-customer Company, so CompanyID probably won't end up being the only attribute of Customer.
So if that's the case, the clear choice is to have a separate Customer table.
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I'm learning how to design databases, and i've been ask to create the table that will hold this form: Medical History I'm learning to use Django/Python i've already made the markup in HTML and CSS, but I don't think that making each question on the form an column would be the best approach. For example in the family history i've thought of making it a separate table, while in the review of systems i want to make each to be a set.
A pragmatic approach is to define tables based on the following criteria:
1) easy to select data from them (not to obtain many JOINs or convoluted queries that require ORs or strings splitting)
2) easy to understand (each concept maps to one table)
=> usually, normalized structures do the trick here
Of course, above are challenged in high transactional environments (INSERTs, UPDATEs, DELETEs).
I would assume then your case has moderate INSERTs, but more SELECTs (reports).
For Family history section I would normalize everything:
DiseaseType
DiseaseTypeId
Code -- use to separate from a name that can change in time
Name -- breast cancer, colon cancer etc.
CollateralOption
CollateralOptionId
Code -- I would put UNIQUE constraints on Codes and Names
Name -- no, yes, father
FamilyHistory
FamilyHistoryId INT PK IDENTITY -- this may be missing, but I prefer if I use an ORM
PatientId -> FK -> Patient
DiseaseTypeId -> FK -> DiseaseType
CollateralOptionId FK -> CollateralOption
Checked BIT -- you may not define this and have records for Checked ones.
-- having this may put some storage pressure
-- but prevent some "stuffing" in the queries
These structures allow to easily COUNT number of patients with colon cancer cases in their family, for example.
Shortly put: if there is not serious reason against it, go for normalized structures.
I don't see any advantage to perform any design tricks on this data structure. Yes, making a boolean attribute of each of your checkboxes, and a string attribute of each of your free texts, will lead to a high number of attributes in one table. But this is just the logical structure of your data. All these attributes are dependent on the key, some person id, (or at least that's what I assume, as a medical layman). Also, I assume that they are independent of each other, i.e. not determined by some other combination of attributes. So they go to the same table. Putting them on several tables won't gain anything, but will force you to do lots of joins if you query on different types of attributes (like all patients whose mother had breast cancer and who now have breast lumps).
I don't know exactly what you mean by making sets of some attributes. Do you mean to have just one attribute, and encode the sequence of boolean values e.g. in one integer, like 5 for yes-no-yes? Again that's not worth the trouble, as it won't save any space or whatever, but will make queries more complicated.
If you are still in doubt, try to formulate the most frequent use cases for those data, which will probably be typical queries on combinations of these attributes. Then we might see whether a different structure would make your life easier.
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It is very common coming up with the situation where you have multiple tables, let's say Posts, Entries and News and the users can comment in any of them.
The natural way to think this would look like would be something like this:
However, using the parent_type attribute to set up the primary key seems ambiguous to me, because it doesn't really describe clearly to which parent the comment belongs to.
It would seem to me like the clearest way to understand the relationship would be to create an intermediate table for each relationship: posts_comments, posts_entries, posts_news. But then again this doesn't have much sense in terms of database design because you don't really need an intermediate table for a one to many relationship, they are only needed for many to many relationships.
Then maybe a third approach would be to make a different Comment model depending on what it belongs to: PostComment, EntryComment. This would help in understanding better what that comment is for, but I think it is a terrible idea because a comment could just be represented by a single table and you could end up with a bunch of different tables with repeated column while it could just be represented by one table.
So, what is the usual approach for this situation?
I agree that you don't want to create three tables that are almost identical if you can help it. I've seen plenty of databases that do this and it's a pain, because any change is likely to affect all the variations, so you have to make the change three times instead of once, and there are three times as many queries to change. And sooner or later someone will make a change and not know he should make the same change to the other two tables or be in a hurry or forget, and then six months later someone else comes along and wonders if there's a reason why the three tables are subtly different or if this is just carelessness.
But you also don't want to have a column with an ambiguous definition. You especially don't want a foreign key that can refer to different tables depending on the content of a type field. That would be really ugly.
A solution to a similar problem that I used once was to create an intermediate table. In this case, it would be -- I don't know if you have a word that encompasses news, posts, and events, so let me call them all collectively "articles". So we create an article_comments table. It may have no data except an ID. Then news, posts, and events all have a pointer to article_comments, and comments has a pointer to article_comments.
So if you want all the comments for a given news record, it's:
select whatever
from news n
join article_comments ac on ac.iarticle_comments_id=n.article_comments_id
join comments c on c.article_comments_id=ac.article_comments_id
where n.news_id=#nid
Note that with this structure, all FKs are true FKs to a single table. You don't make article_comments point to news, posts, and events; you make news, posts and events point to article_comments, so all the FKs are clean.
Yes, it's an extra table to read, which would slow down queries a bit. But I think that's a price worth paying to keep the FKs clean.
One admittedly clumsy query with this structure would be if you want to know which article a given comment is for if you don't know the type of article. That would have to be:
select whatever
from comment c
join article_comment ac on ac.article_comment_id=c.article_comment_id
left join news n on n.article_comment_id=ac.article_comment_id
left join post p on p.article_comment_id=ac.article_comment_id
left join event e on e.article_comment_id=ac.article_comment_id
where c.comment_id=#cid
and then see which of news, post, and event turns up non-null. (You could also do it with a join to a subquery that's a union, but I think that would be uglier.)
So later. However, you can design model as same below:
#Entity
uid: string
type: string/ (post, new,...)
...
#Comment:
uid: string
entity_id: string
body: String
Other way (only one table)
Entity:
uid: string
parent: string
type: string /post, new, comment, revison,...
metadata: jsonb
Hope useful for anyone!
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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 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.