how to return field based on field= NULL or not

Hi,

In my table, I have fieldA and fieldB. At any point in time, only one of
these fields can have data in it. The other is NULL.

Instead of "select fieldA, fieldB from table", I want it to return either
fieldA or fieldB depends on whether it is NULL or not.

The reason is because I want to use "select
array_to_string(array_accum(field A or field B) ,',') from table.

Is it possible to do it that way?

Thanks,
JB