SQL is considered to be an RDBMS:
Relational Database Management System
In a relational database, data is represented as tuples and are grouped into relations.
SQL is a declarative language. This means that the user declares what data should be stored in the database and the database management system will figure out how to store the data. SQL is an approximation of what Edgar F. Codd described as the relational model for databases, so it can be considered a relational database management system.
MySQL stores rows and columns of data in tables within databases. A relationship happens between tables:
The following tables have a relationship:
|001||Intel Core i7 2600|
|002||Intel Core i5 2400|
|003||Intel Core i3 2120|
The relationship here is that they both have a product identifier. Since these match, it's easy to find out the name of a product purchased at $270.
This is one of the key principles of an relational database management system.
Relations are made between keys. In the example above, the two tables are matched on the product_id fields in the two tables. This is called a foreign key.
A primary key is a unique identifier in a table. In otherwords, each row has a unique way of accessing it. If it is not specified, the MySQL database management system will create a field automatically which uniquely identifies each row but it will not be accessible other than to MySQL.
Keys create constraints on a table.