Caching at wireless access points is a promising approach to alleviate the backhaul burden in wireless networks. In this paper, we consider a cooperative wireless caching network where all the base stations (BSs) are connected to a central controller via backhaul links. In such a network, users can get the required data locally if they are cached at the BSs. Otherwise, the user data need to be assigned from the central controller to BSs via backhaul. In order to reduce the network cost, i.e., the back-haul cost and the transmit power cost, the data assignment for different BSs and the coordinated beamforming to serve different users need to be jointly designed. We formulate such a design problem as the minimization of the network cost, subject to the quality of service (QoS) constraint of each user and the transmit power constraint of each BS. This problem involves mixed-integer programming and is highly complicated. In order to provide an efficient solution, the connection between the data assignment and the sparsity-introducing norm is established. Low-complexity algorithms are then proposed to solve the joint optimization problem, which essentially decouple the data assignment and the transmit power minimization beamforming. Simulation results show that the proposed algorithms can effectively minimize the network cost and provide near optimal performance.