In Oracle Database, what does a CHECK constraint do when defined on a column or set of columns?

Introduction / Context:
This question asks about the purpose of a CHECK constraint in Oracle Database. CHECK constraints are a powerful way to ensure that data in a column adheres to business rules that can be expressed as Boolean expressions. They supplement other integrity constraints such as NOT NULL and FOREIGN KEY by allowing custom validation logic directly in the database layer.

Given Data / Assumptions:

- We are working with table and column definitions in Oracle.
- A CHECK constraint is defined with a condition that must evaluate to TRUE for allowed rows.
- Only one option correctly explains this behavior.

Concept / Approach:
A CHECK constraint specifies a condition that each row must satisfy. The condition is a SQL expression that returns TRUE or FALSE, such as salary > 0 or status IN ('A','I'). When a row is inserted or updated, Oracle evaluates the condition. If the condition evaluates to FALSE, the database rejects the operation. If it evaluates to TRUE or UNKNOWN (when nulls are involved), the row passes the check, subject to other constraints.

Step-by-Step Solution:
Step 1: Recall that the syntax for defining a CHECK constraint includes a condition in parentheses after the CHECK keyword. Step 2: Think of simple examples, such as CHECK (age >= 0 AND age <= 120) or CHECK (gender IN ('M','F')), which ensure data lies within allowed domains. Step 3: Review option A and see that it states that the CHECK constraint enforces a Boolean condition, such as ranges or patterns, which matches the understanding. Step 4: Option B describes NOT NULL, not CHECK, because NOT NULL simply disallows null values. Step 5: Option C suggests automatic index creation, which is not done by CHECK constraints; indexes are separate objects. Step 6: Option D describes something like a read only column, which is not the purpose of CHECK constraints.

Verification / Alternative check:
You can verify by creating a table with a CHECK constraint and trying to insert rows that violate the condition. For example, if you define CHECK (salary > 0), attempting to insert a row with salary equal to zero will result in an error. There is no automatic index creation or prevention of all updates; only updates that violate the condition are blocked.

Why Other Options Are Wrong:
Option B is wrong because NOT NULL constraints, not CHECK constraints, guarantee that a column is never null. Option C is incorrect since Oracle does not automatically create an index for every CHECK constraint. Option D misinterprets the feature by suggesting that all updates are disabled, whereas CHECK constraints allow updates that satisfy the defined condition.

Common Pitfalls:
One pitfall is writing overly complex CHECK conditions that are hard to maintain or that can degrade performance. Another is forgetting to account for null values and how they interact with Boolean logic, since a CHECK condition that evaluates to UNKNOWN may still be allowed. Designers should keep CHECK constraints clear and focused on enforcing meaningful domain rules.

Final Answer:
A CHECK constraint enforces that column values satisfy a specified Boolean condition, such as being within a valid range or matching an allowed set of values.