LeetCode 584: Find Customer Referee

Meta Data

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Difficulty: easy First Attempt: 2025-12-23

• Submissions: Accepted (first try)

Approach

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We want customers not referred by customer 2, which includes those with no referrer (referee_id is NULL).

SQL comparisons have three outcomes, and NULL != 2 is unknown, not true, while WHERE keeps only true rows (SQL Template 2). Writing only referee_id != 2 therefore drops the NULL rows; add IS NULL explicitly.

My Solution

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SELECT name
FROM Customer
WHERE referee_id IS NULL OR referee_id != 2;

WHERE COALESCE(referee_id, 0) != 2 also works, replacing NULL with a value that cannot be 2.

Complexity

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One table scan, O(n).

Mistakes to Avoid

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• Test for NULL with IS NULL; neither = NULL nor != 2 selects NULL rows.

Interview Explanation

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Customers not referred by 2 include those with no referrer, but NULL != 2 is unknown in SQL and WHERE drops it, so the condition is referee_id IS NULL OR referee_id != 2.