So one of the database models that Oracle follows is the relational database management system. Relational database management system, RDBMS. And in that instance, if you look at this, I have an example here of employees that sell goods and services to customers. The employees table. The sales table. The customers table. The thing is, employees don't have a direct relationship with customers. Isn't that true? The way employees know customers is because of, well, the fact that they sell to them. So there is no direct relationship here. However, employees sell to customers. Employees sell to customers, so when you write that sentence out, it turns out that way. Here's another thing. The employees table, let's say has an employee ID and a name. Customers table has a customer ID and a name. The sales table has a sales ID, an empID, and a customer ID. Okay, so the thing is, you are selling to customers, employees are selling to customers. Now in this instance, this isn't very real because I'm not specifying what products are sold because here I have enlisted a products table. Realistically speaking, you will sell products or you will sell services. But for the sake of argument, I'm simply stating that one employee will have one or more sales, right? One or more which is many. That's the one to many, okay? And that one customer, will have one or more sales. One to many. One to many relationship is a very common kind of a relationship. And the way the employees table is related to the sales table is via employee ID. So the sales table has a employee ID. And the employees table has an employee ID. That's a relationship. And the way customers are related to the sales table is via a customer ID. So, I have a customer ID column in the sales table, as well as a customer ID column in the customers table. The thing is, the customer ID column in the customers table would be considered what is called a primary key. In other words, it's a unique identifier for that customer in the customers table. The customer ID, and the customer table cannot be repeated. Customer ID in the sales table can be repeated because one customer is going to buy one or more things. You hope so. Isn't that true? And the employee ID in the employee table is the primary key. Primary key can't be repeated, unique to the employees table. But the employee ID in the sales table can be repeated because one employee is going to have one or more sales. Hopefully a lot more sales, right? So if you really think about it, this whole primary key here for the employee ID. And primary key here for the custID, what does that make these keys? Well, these keys are foreign keys. Foreign keys are related to primary keys usually, but not always in a different table. So the empID foreign key is associated with the employee ID primary key in the employees table. The custID foreign key is associated with the custID primary key in the customers table. Let me show you this how I constructed the statement for this. So here's the SQL statement, okay? And in this SQL statement, here's the customers table right here. Here's the employees table right here and here's the sales table right here, okay? You notice the empID here, the custID here, and the sales ID here. You also notice the primary key here is empID. The primary key here is custID, and you notice the primary key here is sales ID. However, here are the foreign keys. Notice those? There are two of that foreign keys that exist in the sales table. The foreign keys only exists in the sales table because these foreign keys, foreign key of empID is associated with the employees table. And the foreign key of custID is associated with the customers table. You notice custID, empID, you notice that references, customers in custID and references employees empID? Okay? So this right over here is talking about these columns here, and these columns right over here, these columns right over here. EmpID and custID references are in the employee and the customers table, respectively. Primary key, foreign key. These are called constraints, and they allow you to do something called referential integrity or RI. Referential integrity with these primary and foreign keys, the way I have it right inside of the sales table, defined right there. Means that the sales table cannot have a customer number that doesn't exist in the customers table, okay? And cannot have an employee that doesn't exist in the employees table. That's the referential integrity or RI that is provided by having these constraints, these foreign key constraints. Okay, so keep that in mind. I'm going to show you a demonstration of how this works as well.
