Hallo All,

I'm trying to write a procedure creating a trigger for change history.
The procedure has to be in plase because fields to a table may be
added at runtime. I let users create fiels where the field name is
given by the system and the apparent title is put in the field
description. this works fine; however, i would like to have the
correct mapping on the change history table (users can see that table).

I defined the procedure as follows - i my eyes the correct way:

...
declare variable TABLEN varchar(255);
declare variable TRIGRN varchar(255);
declare variable FName varchar(255);
declare variable FType varchar(255);
declare variable Fcapt varchar(255);
...
for select
trim(RF.RDB$FIELD_NAME),
trim(T.RDB$TYPE_NAME),
cast(substring(RF.RDB$DESCRIPTION from 1 for 20) as varchar(255))
from
RDB$RELATION_FIELDS RF inner join
RDB$FIELDS F on F.RDB$FIELD_NAME = RF.RDB$FIELD_SOURCE inner join
RDB$TYPES T on T.RDB$TYPE = F.RDB$FIELD_TYPE and T.RDB$FIELD_NAME =
'RDB$FIELD_TYPE'
where
RF.RDB$RELATION_NAME = :TABLEN
into
:FName,
:FType,
:Fcapt
do
...

The error says "cannot convert to string", but the line:
cast(substring(RF.RDB$DESCRIPTION from 1 for 20) as varchar(255))
returns very well a field varchar as some test have shown. but as soon
as i want to use the variable :Fcapt i get the error.
Can someone explain that ?