In Oracle Database, what does a NOT NULL constraint ensure when applied to a column definition?

Introduction / Context:
This question tests understanding of the NOT NULL constraint in Oracle Database. NOT NULL is one of the simplest but most important constraints for enforcing data integrity. It is often used in combination with other constraints such as PRIMARY KEY and UNIQUE to ensure that key columns always have meaningful values.

Given Data / Assumptions:

- The question is about a NOT NULL constraint on a column.
- A NOT NULL constraint controls whether null values are allowed.
- Only one option correctly states what NOT NULL enforces.

Concept / Approach:
A NOT NULL constraint is a column level constraint that tells the database that null values are not permitted for that column. Whenever you try to insert or update a row, the database checks whether the column has a non null value. If a null is provided, the statement fails. This is different from uniqueness or indexing; NOT NULL simply addresses whether the absence of a value is allowed.

Step-by-Step Solution:
Step 1: Recall that NOT NULL is specified in the column definition, for example salary NUMBER NOT NULL. Step 2: Understand that with this constraint, any insert or update that does not supply a value for salary or explicitly sets it to null will be rejected. Step 3: Review option A, which states that the column must always contain a value and can never be left null. This matches the definition. Step 4: Option B describes a UNIQUE constraint, which ensures no duplicate values but still allows a single null unless combined with NOT NULL. Step 5: Option C incorrectly claims automatic index creation, which Oracle does not do purely for NOT NULL constraints. Step 6: Option D suggests that updates are completely blocked, which is not true; updates are allowed as long as they do not assign null.

Verification / Alternative check:
You can verify by creating a table with a NOT NULL column and then attempting to insert a row without specifying that column or explicitly setting it to null. Oracle will raise an error indicating that the column cannot be null. Updating an existing row to set the value to null will likewise fail. This behavior confirms the explanation in option A.

Why Other Options Are Wrong:
Option B is wrong because it conflates NOT NULL with UNIQUE. It is possible to have multiple rows with the same non null value under a NOT NULL constraint alone. Option C is incorrect because indexes are created explicitly or as part of primary key and unique constraints, not for NOT NULL by itself. Option D misrepresents the constraint by implying read only behavior, which is not what NOT NULL provides.

Common Pitfalls:
A common pitfall is forgetting to mark important business key columns as NOT NULL, allowing rows with missing key information. Another is assuming that NOT NULL will prevent duplicate values, when in fact it does not. Designers should combine NOT NULL with other constraints such as PRIMARY KEY when uniqueness and presence are both required.

Final Answer:
A NOT NULL constraint ensures that the column must always contain a value and can never be stored as null for any row.