What is NULL?
NULL
Indicates that there is no value. It is different from 0.
What it means
A value of 0 means “I counted it and it wasn’t there,” and an empty value means “I couldn’t count it, I don’t know.” If you treat the two as the same, what you don't know turns into what you don't know.
When summing or averaging, if you put empty values as 0, the number will be smaller than it actually is. If there are empty values, they must be marked and notified separately.
At the pen factory
On the 21st day, the wire on the sensor became loose and gave blank bad readings for two hours. The screen added it to zero so the bad total appeared to be 9 instead of 14.
Why it matters
“There were no defects” and “Could not count the defects” are completely different things.