A salt undergoes hydrolysis in aq.soln.
In case of NaSO4, which is a salt of strong acid and strong base, strong ionic bonds are present between Cation and anion.
So,it may not undergo hydrolysis, and as the pH of water is neutral value, the soln. is also neutral. Even if it undergoes hydrolysis, strong base and strong acid is formed. So, the whole soln. is neutral.
However,in case of NaCO3, it undergoes hydrolysis to form a strong base and weak acid. So, the base completely neutralises the acid and remains in excess. So, the aq.soln. of NaCO3 is basic in nature.