Composer: Nobuo Uematsu / Composition: Jenova (Final Fantasy VII)
This musical composition is very complex. The piece initially starts of in the key of F major, which has one accidental (B-flat). The song quickly switches to the relative minor key (D minor) in order to change the mood of the song from happy to sad. To make things more interesting, the key signature changes once more to the key of A-flat major to give it an uplifting feel with its four accidentals (B-flat, E-flat, A-flat, and D-flat).
Suffice to say, the relation between major and minor keys has a lot to do with the emotional mood of a song.