Finding rows that do NOT meet a specified condition In standard SQL, which construct is most appropriate to return rows that have no matching rows satisfying a condition in a related set?

Introduction / Context:
It is common to ask for rows that do not have a related record, such as customers without orders or products without prices. SQL provides several ways to express anti-matching; choosing the right one affects both correctness and performance.

Given Data / Assumptions:

• A main table (outer query) and a related table (inner subquery) exist.
• We want rows where no qualifying related rows exist.
• NULL semantics can affect IN/NOT IN behavior.

Concept / Approach:

NOT EXISTS expresses anti-matching cleanly: it is TRUE when the subquery returns no rows meeting the specified condition. Because EXISTS/NOT EXISTS respond only to row presence (not values), they avoid pitfalls where NULLs in subquery results can cause unexpected filtering (common with NOT IN).

Step-by-Step Solution:

Verification / Alternative check:

Replace NOT EXISTS with LEFT JOIN ... WHERE o.customer_id IS NULL as an equivalent anti-join; both convey the same intent when written correctly and with appropriate NULL handling.

Why Other Options Are Wrong:

• A: EXISTS returns rows when a match does exist, not when it does not.
• B: Doubling NOT EXISTS is unnecessary and creates confusion.
• D/E: Incorrect or error-prone approaches, especially with NULLs in IN lists.

Common Pitfalls:

• Using NOT IN with a subquery that returns NULLs, which can exclude all rows.
• Forgetting correlation, leading to global NOT EXISTS that returns unintended results.

Final Answer:

NOT EXISTS