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COThis is a classic SQL interview trap question.
Why is NULL = NULL false but IS NULL true in SQL?
1️⃣ NULL means “unknown”
In SQL, NULL is not a value.
It represents missing or unknown data.
⸻
2️⃣ Comparison with unknown is unknown
When you write:
NULL = NULL
You are comparing unknown with unknown → result is UNKNOWN, not TRUE.
3️⃣ SQL uses three-valued logic
SQL has:
•TRUE
•FALSE
•UNKNOWN
NULL = NULL → UNKNOWN → treated as FALSE in conditions.
4️⃣ IS NULL is a special check:
column IS NULL
This does not compare values.
It checks whether the value is missing → returns TRUE.
🎯 Interview takeaway
= compares values
IS NULL checks existence
NULL is not equal to anything — not even itself.
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