Hey, everyone. Welcome back to topic two in our database class. In this video, part five, we're going to focus on candidate keys. And a candidate key is called a candidate because it is, it's almost like an election, right? It potentially could become the primary key. So if we're, I don't know, like in Britain, they selected themselves a new prime minister. Okay. So there were several candidates to become the prime minister and ultimately one of them is selected to become the prime minister in this case, the primary key. A primary key then is the key within the table that we have labeled or chosen or selected as the basis for uniquely identifying the the rows within that table. Right. So let's consider, for example, some possible ways that we might uniquely identify the people at a university. So we have in our world something called a campus wide ID that uniquely identifies everyone. But so what are some other options that we could use? What are some other things that are unique that we potentially could use to identify every person at the university? I think if you start thinking about this, you'll start seeing primary keys everywhere. Like an email address is unique. Right. Every single email address is unique across the entire internet. Right. Like my email address at the university is D soper at fullerton dot edu. Nobody else in the world has that email address. It is absolutely unique to me and my role as the university. And the design of the internet precludes anyone else from having that. Right. It would be impossible to route email if there was more than one email account with the say exactly the same name. So here's the possibilities these days we're getting to a point where people have their own phones. Right. So it might be possible to uniquely identify someone based on their phone number now in the past that was not true because you would have a say, for example, one phone shared by a family. But, Tony, this has become a more feasible option. There are some other ones like most countries have some sort of national ID number in the United States. We use the social security number for that purpose. Other countries have different types of national ID numbers. But for people that are citizens of or legally allowed to work in the United States, they will have a social security number to uniquely identify them from everybody else. Right. Other possibilities. Maybe you have a driver's license number. So this could possibly be used every one of those is unique other common things that we have out there. I don't know like license plates. If you drive, you will have a license plate number on your car or your vehicle that will be unique from everybody else. So we could go on and on with these things try to come up with more and more possible identifiers. So let's say that we have this set of candidate keys. Each of which potentially could be used to uniquely identify every individual at university. So these are our candidates. These are our candidate keys. And then what we need to do is choose one of them to serve as the primary key. So we want to pick one of these that we will ultimately use throughout our entire database system and all of the apps that rely on that database system to uniquely identify individuals at our university. So at this point we have a design decision to make and we have to look at these options that we've come up with. And we say, okay, which one of these is best or which one of these would work? We could say about license plate number, not everybody owns a car and some people own multiple cars. So if you do not own a car, you won't have a license plate number. If you own multiple cars, then you'll have several of these. So this might not ultimately work. You may also have, let's say that you have a spouse and you both own the same car together and you're both say employees at the university. In that case, we realistically could not use the license plate number of your car to uniquely identify individual humans. But about a driver's license number. Again, not everybody has a driver's license. I think most adults do, but there are a lot of people out there like me who don't. I can't see well enough to drive a car so I do not have a driver's license and it's probably a good idea that I don't. I have a state ID card, but it does not entitle me or qualify me to drive a vehicle because that would be a disastrous. It would be just a big problem. So there are people who do not drive like me and hence we could not realistically use a driver's license number. As a basis for uniquely identifying people at the university. What about a national ID number like a social security number? Universities have lots of, let's say, international students that may not have a social security number. So we can't realistically use that either. phone numbers. Again, we conceivably could make an assumption that everybody has their own phone number. But there are certainly cases where people don't have phone numbers. Some people don't have phones or they just have a phone at home and choose not to use like a mobile phone. So we can't use phone numbers to uniquely identify people. An email address would work. But there are some reasons why we might not want to use an email address. Like it's going to be typically a long string of text which might not be very computationally efficient. And email addresses can be changed as necessary. So for example, if your email address were compromised or came affiliated with something that you didn't want it to be connected with, the university could conceivably issue you a new email address. So they're a little more mutable, a little more changeable than other things. So we can find reasons not to use a lot of these other things. And what we settle on then is we choose the campus white ID. It's just a number that was invented for the purpose of uniquely identifying individuals who are in some way related to the university. They could be faculty, staff, administrators, students, whatever it may be, they have a campus white ID. So in this case, you can see we've gone through this process of considering our various options. And that we've chosen from among these candidates. One of them we've identified as the superior candidate and have elevated it to the status of primary key. That is, we will use the campus white ID in this scenario as a basis for uniquely identifying everyone at the university. But yeah, once you start to view the world through this lens, you see it everywhere. License plate numbers are primary keys, drivers, license numbers, checking account numbers, credit card numbers. When you get an airplane ticket, you get a confirmation number, primary keys. They're just used to uniquely identify rows within tables. That's their purpose.