Original subtitles

‫So we ended on cardinality and we've looked at a lot of the relational model, we've seen so much of

‫it, but yet we haven't hit the main point of what makes the relational model the relational model,

‫and that is the ability to link relationships between different types of data.

‫If we look over here, we can see something called the primary key.

‫And so the primary key, in essence, is a key.

‫Now, you may be thinking, well, yeah, I can read two primary key, but what this is, is something

‫that uniquely identifies your data.

‫We've always seen this in most of the tables that I've shown you.

‫We've seen the ID field there and we've seen it uniquely identify each and every one, two, three,

‫four, five.

‫It is a unique identifier, uniquely identifies each and every role.

‫And why is that important?

‫Well, uniquely identifying each and every piece of data means that we can now know for certain the

‫person with I.D. one is mobily.

‫Now, it's great, I'm number one, I'm not trying to say that, but what I'm trying to say here is

‫that because of the I.D. being the primary key, primary key being the term that we use to say this

‫is the thing that's uniquely identifying each and every role because it is the primary, we can now

‫use it and we can say, well, hey, I have some other data over here.

‫And it's kind of linked to this is kind of linked to Mobeen.

‫And so I can use his ID, right?

‫I can use his I.D. and I can say, oh, this is linked to MCGEENEY.

‫And yes, you can.

‫That is the vital part of the relational model, the ability to draw relationships.

‫And so how would that look?

‫Well, let me show you.

‫There is also something called the foreign key, and the foreign key is just this other terminology

‫for saying, well, if the primary key is uniquely identifying my data in place, if I am going to reference

‫data from somewhere else, well, I'm going to reference it by that primary key because like I said,

‫I want to uniquely identify MCGEENEY.

‫So Mobeen is number one.

‫Well, if I went into the manager ID, I can see here, M1, M2, M3.

‫And so if I'm going to link that relationship, if I'm going to say, well, you know what, Clive Bowie

‫is the manager of Moe, well, then I'm going to have to do that in a certain way.

‫So we see over here in Orange the manager ID being the primary key of managers and one and two and three.

‫And so over here we see the primary chemokine one.

‫And the manager of Mobily.

‫Is done by drawing a relationship to manage, right?

‫And the way we do this is by injecting a column, there is a new column in this table in the sole purpose

‫of this column is to manage that relationship is to say, hey, well, Mo.

‫Is managed by anyone, so a foreign key references the primary key of a different table.

‫Let me repeat that again.

‫A foreign key is a key that will reference the primary key, the unique identifier of a different table,

‫therefore allowing a relationship to be formed that is solidified in stone, more so than my fiance

‫and I's marriage.

‫I'm kidding there, of course.

‫But that being said, with this, what you basically get is a relationship and a relationship that is

‫warranted by the fact that is linked to a key and that key is unique.

‫That key cannot be changed.

‫That key cannot be deleted because if it is deleted, then it would cause issues throughout the system.

‫We'll get into that part later.

‫But that being said, what we need to know now is because we are referencing a unique key and because

‫we are injecting this column that says Manager Idee.

‫In the same table as the employees will now we know for certain that Mo, its manager, is Clyde Bowie,

‫and that is how we link relationships.

‫We link relationships by primary keys.

‫And once you've created a relationship, a primary key.

‫Where it is injected becomes the foreign.

‫So now that we know all of that, we know the core of the relational model, we know the essence of

‫why the relational model is the way it is, this unique way of trying relationships is what makes it

‫special.

‫Amazing.

‫Well, now we know the relational model.

‫Now we know all of this stuff.

‫Let's look at how these databases can be used.