Dynamic adaptive streaming has emerged as a popular approach for video services in today's Internet. To date, the two important components in dynamic adaptive streaming, video transcoding that generates the adaptive bitrates of a video and video delivery that streams the videos to users, have been separately studied, resulting in a huge waste of computation and storage resource due to producing and caching different versions of videos regardless of their demands. We conduct extensive measurement studies of video sharing systems, including an IPTV service which streams regular, professionally made videos and an instant video clip sharing service which provides extremely short user-generated videos, as well as the availability of computation resource in conventional content delivery networks (CDNs). Based on the measurement insights, we propose an online joint transcoding and delivery approach for adaptive video streaming. We formulate optimization problems to enable high streaming quality for the users, and low computation and replication costs for the system. In particular, our strategy connects video transcoding and video delivery based on users' preferences of CDN regions and regional preferences of video versions. We analyze hardness of these problems and design distributed solutions. Extensive trace-driven experiments further demonstrate the superiority of our design.