[nycphp-talk] Need help understanding NULL
danielc at analysisandsolutions.com
Sat Aug 29 16:20:48 EDT 2009
> but the concept of a ???a missing unknown value???
> makes my brain hurt.
Let's say you wrote a piece of software that asks users a set of
questions. Before a user responds to a given question, its answer field
is null. When they answer the question, its field gets assigned the
actual value chosen by the user. When the user finishes, sometimes they
have skipped some questions, so they will remain null.
This is useful, in part, for statistical purposes, since the null record
won't be added into averages, etc. For example, you had three surveys
filled out. Two people actually filled in their ages (10 and 20), if you
do an average when using nulls, you'll get an average age of 15. But if
you use 0's instead of nulls, the average age will incorrectly be 10.
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