add subtitle to title, but only if PRESENT!?

A question about tagfield operation by actions (i'm new in this actions Things, but really happy about this usefull tools).
Background: as BPM-Studio User all my mp3's have Artist, Title, Subtitle, BPM. Sometimes additional Album and/ore Comment.

As NEW User of algoriddim's djay for ipad there is NO possibility to show other tagfields then only artist and title. Thatswhy i have to put my "version" (=subtitle) Information into the title-Field.

I wannt to do it that way (e.G.):
from:
File: MC Hammer - Can't touch this (remix).mp3
Artist: MC Hammer
Title: Can't touch this
Subtitle: Remix

to:
File : MC Hammer - Can't touch this (remix).mp3 [stay's as is/was]
Artist: MC Hammer
Title: Can't touch this - Remix
Subtile: Remix

=> The Subtitle should be shown as "- xyz" but only if subtitle is present.
I no subtitle is present, then Tagfield title should only show the title as is/was before action (without adding "- "!)

Big thanks in advance.

Format the tagfield TITLE with ...

%TITLE%[' - '%SUBTITLE%]

DD.20140112.1558.CET

Create an action of the type "Format value" for TITLE
Enter as format string: $if(%subtitle%,%title% - %subtitle%,%title%)

Tried this and works very Good for me. THANKS a lot! DANKE!