SQL: Using NULL values vs. default values

Question

What are the pros and cons of using NULL values in SQL as opposed to default values?

Answer 1

Nulls and default values are different things used for different purposes. If you are trying to avoid using nulls by giving everything a default value, that is a poor practice as I will explain.

Null means we do not know what the value is or will be. For instance suppose you have an enddate field. You don't know when the process being recorded will end, so null is the only appropriate value; using a default value of some fake date way out in the future will cause as much trouble to program around as handling the nulls and is more likely in my experience to create a problem with incorrect results being returned.

Now there are times when we might know what the value should be if the person inserting the record does not. For instance, if you have a date inserted field, it is appropriate to have a default value of the current date and not expect the user to fill this in. You are likely to actually have better information that way for this field.

Sometimes, it's a judgement call and depends on the business rules you have to apply. Suppose you have a speaker honoraria field (Which is the amount a speaker would get paid). A default value of 0 could be dangerous as it it might mean that speakers are hired and we intend to pay them nothing. It is also possible that there may occasionally be speakers who are donating their time for a particular project (or who are employees of the company and thus not paid extra to speak) where zero is a correct value, so you can't use zero as the value to determine that you don't know how much this speaker is to be paid. In this case Null is the only appropriate value and the code should trigger an issue if someone tries to add the speaker to a conference. In a different situation, you may know already that the minimum any speaker will be paid is 3000 and that only speakers who have negotiated a different rate will have data entered in the honoraria field. In this case, it is appropriate to put in a default value of 3000. In another cases, different clients may have different minimums, so the default should be handled differently (usually through a lookup table that automatically populates the minimum honoraria value for that client on the data entry form.

So I feel the best rule is leave the value as null if you truly cannot know at the time the data is entered what the value of the field should be. Use a default value only it is has meaning all the time for that particular situation and use some other technique to fill in the value if it could be different under different circumstances.