FB is a little "dumb" when it comes to reconciling data types in a UNION.
It has already been suggested to make it smarter in this area.

Use a Cast

CAST('' AS VARCHAR(30)) as condition

Obviously, cast it to match the datatype of the condition field in the
database.

rajsubramani wrote:

>I'm using firebird 1.0.
>
>When I try to do
>
>select book_id, price, '' as condition from Book
>UNION
>select bk.book_id, price, condition from Stock sk, Book bk where
>bk.book_id = sk.book_id
>
>I keep getting
>
>SQL error code = -104
>Invalid command
>Data type unknown
>
>
>This works with many other databases that I have tried.
>
>Does UNION not work with Firebird.
>
>Thanks for the help.
>
>Cheers
>-raj
>
>
>
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--
Thomas Miller
Delphi Client/Server Certified Developer
BSS Accounting & Distribution Software
BSS Enterprise Accounting FrameWork

http://www.bss-software.com
http://sourceforge.net/projects/dbexpressplus