In MySQL, the NOT NULL constraint is used to restrict a column to NOT accept NULL values.
The NOT NULL constraint allows a column to always contains a value which means you cant not insert/update a record in a table without passing value for that column on which you have applied the NOT NULL constraint.
The below statement doesn't allow to "id" column and "first_name" column to accept NULL values:
CREATE TABLE emplyee
id int NOT NULL,
first_name varchar(45) NOT NULL,