Foreign key column with possible copy of self's key value?

[Matt Houser]

I am creating a table as a hierarchy:

Id - Key, auto increment
ParentId -> FK to self table Id
other data, etc.

The root id has ParentId == NULL.

In order to quickly scan data of all children, I want to also have a column of each child pointing to the RootId node.

This works fine, except I want to include the root node in this scan. So is there an easy way to have the default value of RootId column be a copy of the value automatically generated into Id?

I know I could make it nullable, then do an UPDATE after the fact, but I would like to avoid this.

Thanks,
...Matt