Welcome back everyone. In this video, we’re going to continue talking about various types of entities, and subclasses, including union types. This is also somewhat of a subclass idea. Generally speaking with Union types, sometimes relationships are mutually exclusive, like we showed just a little bit ago, that can involve all sorts of things, right? Folders, with files, users groups, all of these things are mutually exclusive, right? A folder can be owned by a user or a group. But how would you prevent a folder from being owned by both types? Right? How would you prevent a folder from being owned by a user and a group, but it’s usually only only owned by one, right? It’s either owned by a particular user or owned by a group, but not both. So this is like a subclass. This is like a subclass. And this is a really big, a big representation here. So I’m actually going to switch screens here. And we are going to take a look at this inside of Lucid Chart. So this is a user folder and group. So in its base form, it’s going to be very difficult to actually represent the mutual exclusivity here. So a user can have zero or more folders, and a group can have zero or more folders. And here again, here, I’m combining some of my techniques here, right, here’s a self referencing table. So a folder belongs to another folder. And so that parent ID is indeed another folder. So here’s our self referencing fact. And then we have a name for it and group ID and user ID.
So how do we represent the mutual exclusivity here. So a better way of actually representing this because there’s, it’s, there’s really no way of representing or preventing a user and a group from owning a folder here. So down here, I have a much larger representation of this along with some check constraints. So now along with the user, and group, I have an owner. So I separated out the the information into an owner. And that owner is kind of like a subclass, right? We’re, we’re, our user and group are kind of like sub classes to the base class owner, right, just like what we did with the ER style diagram here, you can kind of actually see general representations that we saw with employees, right. So think of this as owner being employee, part time employee, salary, employee and employee type. So I have the same general structure here. But now I’m just combining a bunch of other things along with it, because now I have the multi way relationship here between user folder, group and owner. And then I also have a self referencing relationship here with the folder ID with parent ID. But now with this kind of structure, I can enforce the owner type, I can enforce owner type. So I can make sure that, you know, only F if it’s owned by user that only exists here on the user table, if it’s owned by a group that only exists in the group table. But then down, but then I can now enforce exclusivity, because the owner is indicated by only one single field, right? It’s indicated by one single field before, right.
Before I had group ID and user ID as part of the folder, right? What should our properties right? But the ownership now, instead of having both included US foreign keys here, only the owner ID is included, only the owner ID is included. And so we enforce mutual exclusivity by allowing only one owner type ID here for what the same owner so owner type and Owner ID are unique together. And so only one owner can exist for a folder now, through this process. Only one owner can now exist through this process. So I’m going to include as part and Canvas I’m gonna include these images as well as part of This lecture. So please do take some time to kind of look through this. And please ask if you have questions. This is more of one of our more complicated UML diagrams that we have done so far in terms of relationships. So, this here is kind of a mixture of er, we have multi way relationship and self referencing. So, those are the types of relationships here that I’ve represented. And so, and again, remember this, this one here on the bottom is the general better approached represent the idea that we were trying to do with this with with this. So, this up here, I can’t actually enforce everything that I need to properly as far as the relationship goes. But down here, I can actually enforce that relationship in the database itself, without having to rely on application logic.