Comment by techiferous
9 years ago
It's the difference between "this value is unknown" and "this value is known and empty". It's similar to why JavaScript has both null and undefined.
Here's an example. Let's say a user is taking a survey and the last question is: "Please share any additional thoughts if you have them." A null could mean the user did not answer this question, probably because they haven't gotten to it yet. An empty string could mean the user was asked this question and actually didn't have anything to say.
That's basically the only example I could think of: tracking if someone filled out a form field.
But in that case how can you tell the difference? I mean if they never clicked in the field or tabbed into it and submitted the form is that meaningfully different from they clicked into it and didn't type anything?
I can think of ways to work around it (keep a boolean for 'was filled out', a number that means 'they got up to field 17', etc.) but I'm still not sure you can draw a conclusion of 'they skipped it' vs 'they didn't enter anything'.
You're assuming all the questions are being presented to the user at once. Imagine a UI that presents questions to a user one-by-one. If the user bails out of the survey early, the rest of the survey answers can be null.
In that case you can easily keep track of what step they got up to in the wizard. It's a different way to store the data, but would work just as well.
I think the null vs empty argument works better when there are 40 boxes on screen at once. But again I'm not sure what semantic difference you'd find between 'they left it empty' and 'they never clicked into it'.
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To be fair, this would often be modeled by omitting the field completely. The lack of the field suggests the user hasn't gotten that far. The presence of the field with a NULL value means they didn't provide anything.