KEYS

Keys are used to uniquely identify rows in a table. There are several types of keys in SQL, including:

• Primary Key: A primary key is a column or a set of columns that uniquely identifies each row in a table. A table can have only one primary key, and it cannot contain NULL values.
• Foreign Key: A foreign key is a column or a set of columns that refers to the primary key of another table. It is used to establish a relationship between two tables.
• Candidate Key: A candidate key is a column or a set of columns that can uniquely identify each row in a table. A table can have multiple candidate keys, but only one is chosen as the primary key.
• Surrogate Key: A surrogate key is an artificial key that is used to uniquely identify each row in a table. It is typically an auto-incremented integer and is not derived from the data itself.

flowchart TD
    A["KEYS"] --> B["Primary Key"]
    A --> C["Foreign Key"]
    A --> D["Candidate Key"]
    A --> E["Surrogate Key"]

Example:

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    department_id INT,
    FOREIGN KEY (department_id) REFERENCES departments(id)
);

In this example, the id column is the primary key for the employees table, ensuring that each employee has a unique identifier. The department_id column is a foreign key that references the id column in the departments table, establishing a relationship between the two tables.