It seems to me that SUBSTRING does not work in triggers on strings in
UNICODE_FSS character set.

I have a table TBL with 2 columns:
STRL varchar(120) character set UNICODE_FSS ;
STRS varchar( 20) character set UNICODE_FSS ;

STRS serves for indexing. So I set up trigger before insert:

NEW.STRS = SUBSTRING(NEW.STRL FROM 1 FOR 20);

This work perfect for strings STRL that do not contain national
characters. But STRL containing for example hungarian or czech
characters causes conversion or overflow error and new record is not
inserted.

When I deactivate trigger and set up value for STRS manually (
INSERT INTO TBL (STRL, STRS) VALUES(MyValue, SUBSTRING(MyValue FROM 1
FOR 20) );
) new record is inserted as I expect.

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Have you encounter such problem too?
Am I doing something wrong?
Any suggestions?

Thank you for helping me.