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
sometimesreturns 0 rows instead of 1.  This is a problem since the column I'm trying to update is set to refuse nulls. 
Here'sa sample:<br /><br /> update myTable set myColumn = (Select altColumn from altTable where altColumn != 'XXX'
limit1) where myColumn = 'XXX';<br /><br /> MyColumn cannot accept nulls, but sometimes "Select altColumn ..." returns
0rows, and thus, the query fails.  <br /><br /> Is there a way to set a default value to be inserted into myColumn if
andwhen "select altColumn ..." returns zero rows?<br /><br /> Mark<br />