When to use NULL in MySQL tables

Question

I appreciate the semantic meaning of a NULL value in a database table, different from both false and the empty string \'\'. However, I have often read about performance pro

Answer 1

The empty string should not be used in place of NULL. NULL represents nothing where as the empty string is something, with nothing inside. NULL will always be false when compared to another value (even NULL) and NULL will not be summed in the COUNT function.

If you need to represent unknown information there is no substitute to NULL.

Answer 2

As @ForYourOwnGood said - Null should be used for "unknown" information. For example: If you have a lot of fields that customer should fill-in on registration and some of them are optional. For some reason you might want to reserve an ID for that particular customer and since you don't know if the optional fields are a real choice by the customer to be left empty you should set them as NULL i.e. "unknown" when you first save the row. If the customer submits the form, passes all your validation and then you save the information, then you know for fact that the optional field is left empty by intention.

This is just a good case of using NULL.

Answer 3

The main benefit, of course, is the semantic meaning of NULL, which you mentioned.

In addition to that -- and it may depend on your storage engine, as always, check the documentation -- but in at least some databases, NULLs take up a lot less room than a regular value. For example, if you have a "varchar" column declared to be 20 characters, and it's rarely filled in, you can save a lot of disk space by making it NULL instead of an empty string.

I have never heard of any performance issues with using NULLs, one the opposite. I've heard of people mucking up their counts because they counted NULLs wrong, but never performance. If that's a real thing, I would love to hear about it!

Answer 4

Generally, if an attribute is required, it is defined as Not NULL and if it may be omitted it is defined as nullable.

Answer 5

The meaning of a NULL column is more or less "doesn't apply in this context". I generally use NULL columns in two cases:

• If the field doesn't apply (let's say you have a boolean column is_thirsty and you add two datasets. One human and a stone. In case of the human, you set is_thirsty to either true or false, whereas in the case of the stone, you'd probably set it to NULL.
• If I need to flag something and store some data with the value. Like an inventory close date, which you'd use to a) specify that the inventory cannot be changed any more and b) to specify when the inventory was closed. Instead of having two columns (closed_at and is_closed), I just create the closed_at column and set it to NULL if the inventory set can still be changed, but set the date once it's closed.

Basically it boils down to the fact that I use NULL when the emptyness of a field has a different unique semantic than just an empty field. The absence of a middle initial is just that. The absence of a closing date has the meaning of the inventory set still being open to changes.

NULL values can have nasty side effects and they will make life harder for you to add data to the table and more often than not, you can end up with a mish-mash of NULL values and empty strings for example.

Also, NULL is not equal to anything, which will screw queries all over the place if you are not very careful.

Personally, I use NULL columns only when one of the above two cases applies. I never use it to signify empty fields when the emptyness has no meaning other than the absence of a value.