In SQL, which operator is used to test whether a column contains no data (a missing or empty value)?

Introduction / Context:
Handling missing data is a fundamental concept in SQL. Databases often store unknown or unavailable information as NULL, and SQL provides specific operators to test for the presence or absence of these NULL values. This question checks whether you know the correct operator used to test for the absence of data in a column, which is essential when writing accurate WHERE conditions and avoiding logical errors in queries.

Given Data / Assumptions:

• We are working with relational tables that may contain NULL values in some columns.
• The question asks which operator tests for the absence of data in a column.
• We interpret absence of data as a column value being NULL, not as an empty string.

Concept / Approach:
In SQL, NULL is a special marker that represents missing or unknown data. Standard comparison operators such as = or != do not work with NULL in the usual way. Instead, SQL defines the IS NULL and IS NOT NULL operators to test for NULL and non NULL values. This is different from logical operators like NOT, which invert a boolean expression, and predicates like EXISTS, which test for the existence of rows in a subquery rather than NULL in a column.

Step-by-Step Solution:
Step 1: Recognize that the phrase absence of data in a column refers to NULL values. Step 2: Recall that the correct syntax to test for a NULL value is column IS NULL. Step 3: Compare this with NOT, which is a logical operator that negates a condition but does not directly test for NULL. Step 4: Compare with EXISTS, which checks whether a subquery returns any rows, and does not directly test whether a column is NULL. Step 5: Conclude that IS NULL is the specific operator used to test for the absence of data in a column.

Verification / Alternative check:
To verify, consider a table Employees with a column manager_id that is NULL for top level managers. The query SELECT * FROM Employees WHERE manager_id IS NULL returns all employees without a manager. If we tried WHERE manager_id = NULL, the result would be empty because standard comparison to NULL does not work. This simple test confirms that IS NULL is the correct operator for detecting missing data.

Why Other Options Are Wrong:
Option A is incorrect because NOT is a logical operator that negates the truth value of a condition, such as NOT (salary > 50000), and does not directly test for NULL. Option C is incorrect because EXISTS is used to test whether a subquery returns any rows, not whether a specific column is NULL. Option D is incorrect because only IS NULL is the dedicated operator for testing the absence of data in a column; the others serve different purposes.

Common Pitfalls:
A common mistake is to write conditions like column = NULL or column != NULL, which will never return rows because any comparison with NULL evaluates to unknown. Another pitfall is confusing NULL with an empty string, which are treated differently by most databases. Developers also sometimes forget to handle NULL values explicitly in conditions, which can lead to unexpected results in reports and business logic. Always use IS NULL and IS NOT NULL when dealing with missing data.

Final Answer:
The operator used to test whether a column contains no data is the IS NULL operator in SQL.