PACK

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 (n-1)-tuple g ∊ ∏_CA (R), we define:

then

For example, consider the relation R₁ in Figure 22(a). The relations R₃ and R₄, where R₃ = P₁ (∏_1,2,3 (R₁)) and P₄(R₃), are shown in Figures 22(b) and (c).

Figure 22a

Figure 22b

Figure 22c