Not Null
In database systems, the NOT NULL constraint is used to enforce the rule that a column cannot have a NULL value. NULL values represent unknown or missing data, and by applying the NOT NULL constraint, database administrators ensure that certain fields must contain a value. This is particularly important for fields that are critical for the integrity of the database, such as a primary key or a field that must always contain relevant data to ensure the application or business process runs smoothly. For example, in a customer database, a customer’s email address or phone number might be required, and a NOT NULL constraint would prevent leaving these fields empty.
https://en.wikipedia.org/wiki/Null_(SQL)
The NOT NULL constraint is widely used in relational databases to guarantee that all necessary information is present before inserting or updating a record. This constraint helps prevent the entry of incomplete data and ensures that all the columns in a table adhere to the expected data integrity. For instance, when designing a table for storing employee records, the employee’s name and employee ID might be required to not be null, while the employee’s middle name or secondary email address could allow NULL values. The NOT NULL constraint is often a part of a broader set of constraints like CHECK and UNIQUE that safeguard the quality and consistency of the data.
https://www.sqlshack.com/sql-server-not-null-constraint/
In SQL, the NOT NULL constraint can be applied during the creation of a table, or it can be added later using an ALTER TABLE statement. It’s a fundamental feature in data management, particularly in systems where the presence of complete data is crucial for functionality. The NOT NULL constraint also plays a role in performance, as eliminating nulls can reduce the complexity of data retrieval and processing, especially in large databases. Therefore, while it is crucial for data integrity, it is also an optimization technique used to ensure that every entry meets the required standards.