Abstract
Semi-structured data are commonly represented by labelled graphs. The labels may be strings, integers, . . . Thus their type is atomic. They are carried by edges and/or nodes. In this paper, we investigate a nested graph representation of semi-structured data. Some nodes of our graphs may be labelled by graphs. Our motivation is to bring the data model in use closer to the natural presentation of data, in particular closer to the Web presentation. The main purpose of the paper is to provide query languages of reasonable expressive power for querying nested db-graphs.