Symphonies are almost always presented by number first and foremost, key indications secondary and subtitles last. So for the Mahler 6th, it's usually designated "Symphony No. 6 in A minor, 'Tragic.'" But in casual conversation, it's just "the Mahler 6th." Less often, people call it "the Tragic," but such folks will usually get sneered at.
I do get rankled when people refer to the Beethoven 9th as the "Ode to Joy." That is simply not a part of its title, nor is it even a subtitle. It's simply the name of the poem that is set to music in the last movement. So "Ode to Joy" should only be used to refer to the last movement, not the whole symphony.
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