Differential privacy enables sensitive data to be analyzed in a privacy-preserving manner. In this paper, we focus on the online setting where each analyst is assigned a privacy budget and queries the data interactively. However, existing differentially private data analytics systems such as PINQ process each query independently, which may cause an unnecessary waste of the privacy budget. Motivated by this, we present a satisfiability modulo theories (SMT)-based query tracking approach to reduce the privacy budget usage. In brief, our approach automatically locates past queries that access disjoint parts of the dataset with respect to the current query to save the privacy cost using the SMT solving techniques. To improve efficiency, we further propose an optimization based on explicitly specified column ranges to facilitate the search process. We have implemented a prototype of our approach with Z3, and conducted several sets of experiments. The results show our approach can save a considerable amount of the privacy budget and each query can be tracked efficiently within milliseconds.