A multilevel relational data model that meets the basic operational requirements for a multilevel database system is described. The model is an extension of the standard relational model, and consists of multilevel relations, which contain classification attributes as well as data attributes; multilevel relational integrity rules, which extend the integrity constraints of the relational model in order to pro vide consistency for data at different access classes, including data that becomes "polyinstantiated," a decomposition method for mapping all multilevel real relations into standard (single-level) base relations; and multilevel relational operators, which perform the functions of their counterparts in the standard relational model, while also labeling derived tuples with access classes. The model is defined in terms of the standard relational model, but lends itself to a design and implementation that offers a high level of assurance for mandatory security.
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