----- Original Message -----
From: "Martijn Tonies" <[EMAIL PROTECTED]>
To: <mysql@lists.mysql.com>
Sent: Tuesday, March 14, 2006 1:28 PM
Subject: Re: Accountability with MySQL
>Should you have a flag for the status "movement complete" ? I would say
yes but instead of a simple checkbox, you could store a date value. That
gives you two pieces of information
a) if the date is null then the movement is not complete.
b) if the date is NOT null then the movement is complete and you know when
it finished.
There, Shawn and I disagree :-)
In case of a very simple database, one could use NULL
as a "flag" or "signal". But in general, I would avoid
storing NULLs.
Basics of database design: store what you know.
Given that NULLs basically means the absence of a value
(eg: unknown), you shouldn't be storing NULLs.
Nonsense!!
That's simply wrong. A null means "unknown or not applicable" and is a
perfectly valid value to use in many, many situations. Two classic examples:
1. You have a table containing test scores. Some students get every question
on a given test wrong. Some students don't write the test at all, perhaps
because they were sick. In this scenario, I think the correct thing to store
is 0 for the students who got every question wrong and NULL for the students
who didn't write the test.
2. You have a table containing employee records. One of the columns is
"termination date". What value do you store for a new employee's termination
date? Well, if they are a contractor on a fixed length contract, you could
calculate the date the contract ends; fair enough. If they are a permanent
employee and your area has mandatory retirement, you could calculate the
date they turn 65 (or whatever) and use that. But what if they are a
permanent employee and you don't have mandatory retirement? I would store a
NULL to mean "I don't know right now". Then, if and when they gave notice
that they were leaving, I would change the termination date from NULL to
their last day of work.
--
Rhino
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