OK more testcases for you.

These three selects works fine:

select 'xx' from identtab
union
select 'yy' from nametab
union
select 'zz' from netztab ;

select count(*) from identtab
union
select count(*) from nametab
union
select count(*) from netztab ;

select 'name', count(*) from nametab ;

But this one doesn't work at least within IBConsole:

select 'ident', count(*) from identtab
union
select 'name', count(*) from nametab
union
select 'netz', count(*) from netztab ;

Dynamic SQL Error
SQL error code = -104
Invalid command
Data type unknown
Statement: select 'ident', count(*) from identtab
union
select 'name', count(*) from nametab
union
select 'netz', count(*) from netztab


Guenther