That would be true if all units were always convertible to mm, but we
have volume also, we also have feet etc.. So that the easiest and
cleanest thing to do, with out having to throw the all the functions
into a case statement is an aggregate function that takes the 2 unit
types and then gives back a result.
> Well I don't think you got Alban's suggestion right...
> What he was trying to say was:
>
> - use a regular (not aggregated) function to convert all measures to mm
> - use the normal SUM() to sum those value
> - use another regular function to convert from mm to whatever
>
> select mm_to_m(sum(convert_to_mm(measure))) from a
>
> Which is easier than my solution