Why string + string doesn't work?

When you try to write an expresion like char + char or varchar + varchar, trying to append one string value to another, you'd get an error message like:

expression evaluation not supported

The reason is that + is not SQL standard operator for string concatenation. It is ||. Example:

select first_name||' '||last_name from employee;

Take a note, if one of the columns is NULL, the result will be NULL. Why? Well, NULL means that value is unknown, so if you put together a known and an unknown value, you don't know what will you get as a result. Thus the result is also unknown or NULL.

To work around that problem use COALESCE function.

Also, take a note that, unlike some other database systems, Firebird does follow SQL standard, and NULL is not the same as empty string ''