A computer-implemented method for processing a plurality of data items includes defining a set of one or more categories having a corresponding set of conditions that associate the data items with the categories. A sub-categorization request, requesting to divide a category from among the categories into lower-level categories, is accepted from a user. The data items associated with the category are processed responsively to the sub-categorization request, so as to automatically suggest the lower-level categories.The automatically-suggested lower-level categories are presented to the user, and direction with respect to the automatically-suggested lower-level categories is accepted from the user. A hierarchical structure representing the categories is constructed responsively to the direction, by dividing the category into the lower-level categories. Output based on the hierarchical structure is presented to the user.