What does the 'NOT NULL' constraint indicate in a column declaration?

The NOT NULL constraint in a column declaration in SQL indicates that the column must contain a value; it cannot be left empty or set to NULL. This constraint ensures that every record in the table has a valid value for that particular column.

Key Points:

• Mandatory Value: A column defined with NOT NULL must have a value for every row in the table.
• Data Integrity: It helps maintain data integrity by preventing the insertion of records with missing values in critical fields.
• Error on Insertion: If an attempt is made to insert a row without providing a value for a NOT NULL column, the database will return an error.

Example:

CREATE TABLE users (
    id INT PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(100) NOT NULL
);

In this example, both the username and email columns are defined with the NOT NULL constraint, meaning every user must have a username and an email address when a new record is inserted.