|If you have ever faced this error message
ORA-02262: ORA-%05d occurs while type-checking column default value expression
while trying to alter the column definition of a table to NVARCHAR2 with Default Value just remember :
A 'string' literal is of type VARCHAR2
A N'string' literal is of type NVARCHAR2 !!
So now, you can correct your syntax and alter your table properly like this :
ALTER TABLE mytbl MODIFY mycolumn NVARCHAR2(3) DEFAULT N'A';