> COALESCE(receivables.FIRST_NAME,'')
> || ' '
> || COALESCE(receivables.LAST_NAME, '') as GUEST_NAME

And in case you were wondering why it returns null

(http://ibphoenix.com/main.nfs?
a=ibphoenix&l=;PAGES;NAME='ibp_firebird_null')

Null means:

Does not exist or not defined.

Null <> ''

'Some string' combined with an undefined string will give an
undefined result.

'Some string' combined with '' will give 'Some string'.

Adam