

This operator, as well as the following one, unpack, maps (packs) sets of tuples in R, whose (n−1) components for C_{k} are the same, into a single tuple.
Let be a relation included in the set of relations in the database, with Atr (R)  = n, A ∊ Atr (R) and C_{A} = Atr(R) − {A}. For each (n1)tuple g ∊ ∏_{C}_{A} (R), we define:
then
For example, consider the relation R_{1} in Figure 22(a). The relations R_{3} and R_{4}, where R_{3} = P_{1} (∏_{1,2,3} (R_{1})) and P_{4}(R_{3}), are shown in Figures 22(b) and (c).
Figure 22a
Figure 22b
Figure 22c

