default value for select?

I want to update a column in myTable. The value this column is set to depends on a nested select statement which sometimes returns 0 rows instead of 1. This is a problem since the column I'm trying to update is set to refuse nulls. Here's a sample:

update myTable set myColumn = (Select altColumn from altTable where altColumn != 'XXX' limit 1) where myColumn = 'XXX';

MyColumn cannot accept nulls, but sometimes "Select altColumn ..." returns 0 rows, and thus, the query fails.

Is there a way to set a default value to be inserted into myColumn if and when "select altColumn ..." returns zero rows?

Mark