The study on streaming data is one of the hot topics among the database circle all over the world recently. During the past three decades, conventional database technologies have been well developed and widely applied. Unfortunately, they could not be adopted to handle a new kind of data, named streaming data, which is generated from applications such as network routing, sensor networking, stock analysis, etc. Because of the rapid data arriving speed and huge size of data set in stream model, novel algorithms that only require seeing the whole data set once are devised to support aggregation queries on demand. In addition, this kind of algorithms usually owns a data structure far smaller than the size of the whole data set. The ways to devise such synopsis data structures are introduced. These different approaches are also compared by listing historical works upon two classical problems over stream.