MySQL Integer 0 vs NULL

When using integer columns is it better to have 0 or NULL to indicate no value.

For example, if a table had a parent_id field and a particular entry had no parent, would you use 0 or NULL?

I have in the past always used 0, because I come from a Java world were (prior to 1.5) integers always had to have a value.

I am asking mainly in relation to performance, I am not too worried about which is the “more correct” option.

Answer

Using NULL is preferable, for two reasons:

1. NULL is used to mean that the field has no value, which is exactly what you’re trying to model.
2. If you decide to add some referential integrity constraints in the future, you will have to use NULL.