Thanks Martijn and Adam. The workaround works well. It does help a lot.

1) It is strange that this tip is NOT mentionned in the book of Helen
Borrie "The Firebird book".

2) Nowhere in the Interbase V6 manuals have I seen the possibility to
have a select in place of a column in a select statement. Is it a new
feature of Firebird or something that is not documented? The syntax
mentions that a column can be an "expr" which is "A valid SQL
expression that results in a single value" but it does not define what
a SQL expression is.