Defining string to represent null values in select

Hi,

I can specify the text used to represent a null value in output from copy, but I'd like to do something similar is
selectoutput, eg: all NULL values are represented by NA or NaN. 

I can't find anything in the docs about this.

This could be managed using case statements around all the columns in the query, but is there a simpler way, like
settinga system variable to specify this? 

Thanks,

  Brent Wood