Learning PHP 7

Keys and constraints

Now that we have our main tables defined, let's try to think about how the data inside would look. Each row inside a table would describe an object, which may be either a book or a customer. What would happen if our application has a bug and allows us to create books or customers with the same data? How will the database differentiate them? In theory, we will assign IDs to customers in order to avoid these scenarios, but how do we enforce that the ID not be repeated?

MySQL has a mechanism that allows you to enforce certain restrictions on your data. Other than attributes such as NOT NULL or UNSIGNED that you already saw, you can tell MySQL that certain fields are more special than others and instruct it to add some behavior ...