Hey, everyone. Welcome back. In this video, the final video in topic four, part 12, I'm going to discuss recursive relationships and highlight a few scenarios where they might be usefully applied in entity relationship models. And we'll return to this topic again once we get to our next major topic in the class, topic number five on database design. Just to remind you of this, the idea with the recursive relationship is that we have a table that is related to itself. We call these conceptually, we call them unary relationships in contrast to like a binary relationship, which is a relationship between two tables. So this is a relationship where it has a relationship to itself. That's recursive. And one of the very cool things about recursion is that we can use it to represent hierarchies of any arbitrary degree of complexity. So consider this design here. We have an employee table, and we can see the primary key of the employee table is employee ID. Okay. And we can see that down here, we have a foreign key named manager ID. Now, this is the key insight, the important thing to note here. Manager IDs are employee IDs. Okay. So every manager in a company is also an employee. Okay. So manager IDs are employee IDs. And then what we're doing is we are connecting each employee ID to at most one manager ID, indicating the person who is that employee's manager. So let's just explore this idea very briefly here and graphically. And again, we're going to return to this later, but I want to show you how this is a very amazingly efficient way of recording hierarchical information. So consider what we have here. Employee number one. And remember our design. This is the primary key. And the foreign key is manager ID, but remembering that manager IDs are employee IDs. So this person might be like our CEO, they're going to be the one person in the company where the value of manager ID is null. And that makes sense, right? The CEO does not have a manager. They may report to the board of directors or to their shareholders, but they don't have a direct manager. Everybody else has a manager. Okay. So maybe this next layer down are the other sea level executives in our company. Okay. So this might be like the CIO. And this might be the CFO, like the chief financial officer. And this might be like our, I don't know, CTO or like our chief technology officer. Okay. Now you'll note for all of these, they have their own unique employee ID, which makes sense because employee ID is a primary key. So for employees two, three, and four, the value that we have stored for their manager ID is one, indicating our CEO up here. Right. This is employee number one. Remember, manager IDs are employee IDs. So employee number two reports to employee number one as their manager. So it is employee number three and employee number four. But then we can build it up further so we can take our CFO here, who is employee ID number three, and maybe they have some people that to work for them. They manage some people in our company. So for them, the manager ID will be three. All right. And that points to our CFO, who in turn reports to our CEO. Okay. So you can see with this very simple design, it's just one table. You can imagine we can record information about a hierarchy of any arbitrary complexity. All right. So we could make this person have four direct reports. I had that work for them. All right. And then maybe run out of room, but then maybe this person here has two people that worked for them and so on. And you can, you don't have to make any changes to this database design. Right. This design here works beautifully. So we could be like some of the back in the 1980s, the many American car manufacturers were very inefficiently structured from a managerial perspective. There was one, I think in a case study that I learned about what I was in school that had teen different layers of management. And just think about that. You have like your CEO and then you've got like the next layer below that. Maybe the CFO, CTO, et cetera. And then a layer below that, a layer below that, a layer below that going all the way down to the lowest employee in the company are 17 different layers of hierarchy in contrast at the time, the Japanese car manufacturers who were really starting to produce higher quality products and take over a lot of the American market, typically had about five layers of management between the lowest in person in the company and the highest person. So a much flatter sort of design. But the point is, regardless of how complex your managerial hierarchy is, you can record the entire thing, even if it's extraordinarily complex or extraordinarily simple using this design here.