Abstract. A complement to a database view Γ is a second view Γ ′ which provides the information necessary to reconstruct the entire state of the main schema. View complementation is central in situations in which a view is to be updated, since the complement Γ ′ embodies the information not contained in Γ. In general, Γ may have many complements. In this work, an approach to identifying and constructing optimal relational complements for a class of relational views is initiated. The approach is based upon simple ideas of unique decompositions within the context of the semilattice of views of the base schema.