This Ableton tutorial covers two selection techniques in Live 12's MIDI Note View that help you manipulate groups of notes more efficiently. Once you've selected a group of notes, handles appear at either end of the selection, and dragging them scales the entire group left or right while preserving the relative spacing between notes. Snapping follows the current grid by default, and holding CMD (Mac) or ALT (Windows) disables it for freeform scaling.
The second technique is selection toggling: CMD+Shift+A on Mac or CTRL+Shift+A on Windows inverts your selection, instantly selecting everything that wasn't selected before. Repeating the shortcut flips back, letting you alternate between two complementary groups of notes without reselecting manually.