What is NULL and how do you test for it?
Model answer
NULL means unknown or missing, not zero or an empty string. Comparisons with NULL give unknown, so = NULL never matches. Use IS NULL and IS NOT NULL. Functions like COALESCE(value, default) replace NULL with a default.
Learn this topic
What Is SQL and What Is a Database? in the course SQL for Beginners: Learn Databases Step by Step.
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