Day 4 — Empty values and NULL

NULL is the quiet cause of wrong numbers in reports. Half an hour today saves you a very bad afternoon later.

NULL means "we do not know"

NULL is not zero. It is not an empty string. It means no value was recorded. In our Customers table, Bose Logistics has no Segment.

This never works WHERE Segment = NULL
Comparing anything to an unknown gives an unknown, not "true". You must use IS NULL.

IS NULL

IS NOT NULL

Why this quietly breaks reports

Run these two and add up the row counts.

Two plus five is seven — but there are eight customers. The row with a NULL Segment appears in neither result, because it is neither equal nor not-equal to 'Retail'. It is unknown.

The habit that saves you Whenever a total does not tie out, check for NULLs first. Nine times out of ten in MIS work, that is the cause.

Managers and NULL

The Employees table uses NULL to mean "reports to nobody" — the top of the tree.

Try these yourself

List customers with no Segment recorded.
Count how many customers DO have a Segment.
Find the employee with no manager.
Show all customers, then explain why "= Retail" plus "<> Retail" does not add up to the total.

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