Since FCC's opening for white space (WS) utilization, database-assisted dynamic spectrum access (DSA) has become the de facto solution for the realization of dynamic spectrum sharing (DSS), due to its simplicity and compatibility with commercial off-the-shelf (COTS) devices. It is envisioned that such technology will strongly support the prosperous wireless multimedia networking (WMN) applications with satisfying QoS guarantees in the future. However, how to counter the time-frequency variant property when exploiting the WS spectrum for the provision of these services to secondary users (SUs) still remains a great challenge. In such context, a dynamic secondary access scheme for database-assisted spectrum sharing networks is proposed in this paper. In the beginning, the spectrum requirements of SUs for diverse services are modeled by considering the minimum required service data-rate and spectrum access duration. Afterwards, the spectrum demand evaluation and bidding policy are formulated based on the service classes of SUs. Furthermore, a double-phase (DP) spectrum allocation scheme, which consists of the initial resource allocation phase and resource allocation adjustment phase, is carefully designed for DSA. Finally, extensive simulations are conducted and the results demonstrate that our scheme can increase the spectrum trading revenue and adapt to varying service requirements.