it is correct to write 0,000 or just 0? Can you say me please if it is correct to write $0,000$ or just $0$? I have to complete a table and there are decimal numbers, and I don't know how to do in the case I have an absolute zero.
 A: One typically uses the same number of decimal places for all entries in the table, both for aesthetic reasons and to indicate  the precision of the values if they are computed numerically.
One notable exception is as Laray suggests: zero written as "$0$" will stand out in the table if all other values have several decimal places.
Another exception  would be if different groups are computed differently--for example, a column of population counts might all have no decimal places, while a column of average ages might have a couple of decimal places each.
A: Mathematically they are equivalent. 
However there are conventions about accuracy that make them different. When writing $0.000$ it is basically assumed that those written digits are correct, but there may be errors further down the expansion. That is it means something like $0.000\pm0.0005$ (or perhaps slightly larger error marigin).
On the other hand if you only write $0$ it might be interpreted as meaning exactly that (without any error marigin). However if you have error marigin on the other numbers it's likely that there's some error for the zero values as well and that they are not different from the other values (except they're close to zero). One should therefore probably use the same number of decimals there as for the other values.
Also in tables one should distinguish between the quantity $0$ and that there is no quantity (which is normally indicated by a dash).
A: $0,000$ cannot be taken wrong, it clearly denotes the value $0$. By the way, the same question holds for every trailing zeroes. F.i. $3,45000$ is all right for $3,45$.
Anyway, as said by @skyking, showing the trailing zeroes stresses that they are exact.
