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Database / SQLite Interview questions

What is a NULL value in SQLite?

NULL represents a missing or unknown value — it's distinct from an empty string ('') or zero (0), and it follows SQL's three-valued logic: comparing anything to NULL using = yields NULL (neither true nor false), not TRUE or FALSE.

SELECT * FROM person WHERE age = NULL;    -- returns nothing; use IS NULL instead
SELECT * FROM person WHERE age IS NULL;   -- correct way to check for NULL

This is a common source of bugs for developers unfamiliar with SQL's NULL semantics: a straightforward = NULL comparison never matches anything, since NULL isn't considered equal to anything, including another NULL — the correct check is always IS NULL or IS NOT NULL.

What does the query `WHERE age = NULL` return?
What is the correct way to check whether a column is NULL?

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