Let R(U) be a relational pattern on the attribute set U, X, Y and Z are subsets of U, and z = u-x-y X-Y. The multivalued dependence X →→→ Y Y in the relational pattern R(U) is established if and only if a given pair of (X, Z) values have a set of Y values for any relation R of R(U), and they are only determined by X. ..

Trivial multivalued dependence and nontrivial multivalued dependence;

If x →→→→→ y and z are empty sets, then x →→→→→ y is called trivial multivalued dependence; If z is not empty, it is called nontrivial multivalued dependency.