PostgreSQL NOT NULL constraint: ensuring columns reject missing values

PostgreSQL's NOT NULL constraint ensures a column cannot store NULL values, which represent unknown or missing data, distinct from empty strings or zero. This rule is fundamental for enforcing data integrity in database tables. The constraint is applied during table creation, as shown in an invoice table example where product_id and qty columns require values. Attempting to insert NULL into these columns triggers an error, demonstrating the constraint's enforcement. NULL values are checked using IS NULL or IS NOT NULL operators. NOT NULL constraints are commonly used on primary keys, foreign keys, and essential business fields like emails or quantities. They are often combined with other constraints such as UNIQUE, CHECK, and PRIMARY KEY to provide comprehensive data validation.