Hi Giang,

To be more clear, I would recommend getting a Speedlight for shooting events and proms in low light situations.
Nikon DSLR's like your D90 use i-TTL flash control that is not compatible with Film TTL flash control or the earlier D-TTL flash control. Third party or aftermarket Flash umits or Speedlights may not support i-TTL flash control and others support it to varing degrees of sucess. The Nikon SB-700 and discontinued SB-600 fully support i-TTL flash control. The latest Metz flashes are also fully compatible.
There are a few Sigma flash units as well as Nissen flash units that support I-TTL to some degree depending on the model.
You can ask questions about Flash and Speedlight compatibility in the Speedlight forum.
Good Luck and Enjoy your Nikons!

Best Regards,
Marty