Postgresql trigger set empty string to NULL

I have a table with field "mac" of type MACADDR. Now I would like to treat a situation (probably with trigger?) when some开发者_JS百科body inserts empty string instead of mac address. I would like to turn this empty string to NULL, so postgresql will not complain: invalid input syntax for type macaddr: ""

What I have now in trigger function is this:

IF CHAR_LENGTH(NEW.mac) = 0 THEN
  NEW.mac := NULL;
END IF;

But it does not seem to work. What would you do, if you want to treat this on DB level?

Thank you very much. -Jan

PS: I am a postgresql newbie. But a fascinated one :)

You can't do what you want with a trigger. Your incoming empty string will be parsed and converted to a macaddr (or at least the parsing will be attempted) before the trigger is executed. However, you could write a simple function to convert empty strings to NULL and use that in your INSERT:

create function macaddr_ify(m varchar) returns macaddr as $$
begin
    if m is null or length(m) = 0 then
        return null;
    end if;
    return cast(m as macaddr);
end;
$$ language plpgsql;

And then:

insert into t (addr) values (macaddr_ify(str));

I'd recommend that your client application properly convert empty MAC address strings to NULLs itself though.