NOT NULL

The NOT NULL constraint ensures that a column cannot have a NULL value. This means that every row in the table must have a value for that column. For example, consider a table employees with a column employee_id that should not be NULL. The NOT NULL constraint can be applied as follows:

CREATE TABLE employees (
    employee_id INT NOT NULL,
    first_name VARCHAR(50),
    last_name VARCHAR(50)
);

In this case, the employee_id column must have a value for every row in the employees table.

name	type
employee_id	INT
first_name	VARCHAR(50)
last_name	VARCHAR(50)

In the table above, each row has a value for the employee_id column, as required by the NOT NULL constraint.