The data center network (DCN), wired or wireless, features large amounts of Many-to-One (M2O) sessions. Each M2O session is currently operated based on Point-to-Point (P2P) communications and Store-and-Forward (SAF) relays, and is generally followed by certain further computation at the destination. %typically a weighted summation of the received digits. Different from this separate P2P/SAF-based-transmission and computation strategy, this paper proposes STAC, a novel physical layer scheme that achieves Simultaneous Transmission and Air Computation in wireless DCNs. In particular, STAC takes advantage of the superposition nature of electromagnetic (EM) waves, and allows multiple transmitters to transmit in the same time slot with appropriately chosen parameters, such that the received superimposed signal can be directly transformed to the needed summation at the receiver. Exploiting the static channel environment and compact space in DCN, we propose an enhanced Software Defined Network (SDN) architecture to enable STAC, where wired connections are established to provide the wireless transceivers external reference signals. Theoretical analysis and simulation show that with STAC used, both the bandwidth and energy efficiencies can be improved severalfold.