If the arguments are not as expected (e.g. arrays of different lengths, both scalars, not numeric) then null is returned.
arrayOrScalar1 - arrayOrScalar2
,
the same length as the input array(s)subtract(array(1,2,3), array(0.1,0.2,0.3))
= [0.9, 1.8, 2.7]
subtract(array(1,2,3), 1.0)
= [0, 1, 2]