NULL: the value that is not a value

Step 1 of 110%

Concept · step 1 of 11

What NULL is

NULL means unknown or missing. It is not zero and not an empty string: it is the absence of a value, and SQL reasons about it with three truth values instead of two.

Comparisons give unknown. 7 = NULL is not false, it is NULL, and so is NULL = NULL.
WHERE keeps only true. a row whose condition is unknown is left out, exactly like one that is false.
Ask with IS. phone IS NULL is true or false, never unknown.

SELECT name FROM customers WHERE phone IS NULL;

Linus has no phone on the board. NULL also spreads: 'tel: ' || phone is NULL when the phone is, and so is most arithmetic. Every step in this lesson is a place where that one NULL changes the answer, and where it surprises people who have written SQL for years.

PostgreSQL docs: Comparison operators and IS DISTINCT FROM ↗

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