iszero returns one if its argument is an integer 0, a float 0.0 or a double 0.0 For any other arguments, of whatever type, it will return zero.
iszero(0) -> 1 (Yes it is)
iszero(0.0) -> 1 (Double precision zero also
is a zero)
iszero(atof('0.0')) -> 1 (As well as single
precision floating point)
iszero(1) -> 0 (No, it's not)
iszero('Cifra') -> 0 (neither is this one)
iszero(NULL) -> 0 (nor this one)