Introduction to NULL and IS NULL

In SQL, NULL represents a missing or unknown value. You can check for NULL values using the expression IS NULL. For example, to count the number of missing birth dates in the people table:

FROM people
WHERE birthdate IS NULL;

As you can see, IS NULL is useful when combined with WHERE to figure out what data you're missing.

Sometimes, you'll want to filter out missing values so you only get results which are not NULL. To do this, you can use the IS NOT NULL operator.

For example, this query gives the names of all people whose birth dates are not missing in the people table.

FROM people
WHERE birthdate IS NOT NULL;

What does NULL represent?

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