Its not possible to have a square whose sides and diagonals are countable numbers.
If 'a' is the side of square then from Pythagoras Theorem the diagonal is given by √2×a
As √2 is irrational and 'a' is a countable number and also we know that the product of a countable number and an irrational number is always an irrational number, hence the diagonal cannot be a countable number.
Hence, there is no possible square whose sides and diagonals are countable numbers.