ABS returns the absolute value. The output is positive even if
the input is negative:
ABS(x) = x if x>=0
ABS(x) = -x if x<0
ABS can be useful for testing values that are "close". For example
this query shows each country that has area that is roughly
70 thousand. The value 70000 is the target value, 500 is the "tolerance"
so the test ABS(area-70000)<500 tests that the area is between
70000-500 and 70000+500. That is 69500<area<70500
