A database comprises a plurality of keys representing respective data items stored in the database and respective data tags associated with at least some of the data items. Data tags represent different identifiers or categories among which the associated data items are grouped. The database is arranged in the form of a tree-structured directed graph in which each of the plurality of keys is represented by a series of nodes and arcs defining a path between a root node and a terminal node, each node being linked to at least one other node by a respective arc, respective arcs for a given one of the plurality of keys representing a respective character or characters of the given key. The arcs and the nodes depending from the root node of data items which represent a sequence of characters shared by different keys are combined, and the data tags are associated with the arcs.