Definition of Multivalued Dependencies

Definition of Multivalued Dependencies

A multivalued dependency (often abbreviated MVD) is a statement about some relation R that when you fix the values for one set of attributes, then the values in certain other attributes are independent of the values of all the other attributes in the relation. More specifically, we say the MVD

holds for a relation R if when we restrict ourselves to the tuples of R that have specific values for each of the attributes among the A's, then the set of values we find  among the B's is independent of the set of values we find among the attributes of R that are not among the A's or B's. Still more specifically, we say this MVD holds if

For each pair of tuples t and u of relation R that agree on all the A's, we can find in R some tuple v that agrees:

1. With both t and u on the A's,
2. With t on the B's, and
3. With u on all attributes of R that are not among the A's or B's.

Note that we can use this rule with t and u interchanged, to infer the existence of a fourth tuple w that agrees with u on the B's and with t on the other attributes. As a result, for any fixed values of the A's, the related values of the B's and the other attributes appear in all possible combinations in different tuples. The following figure suggests how v relates to t and u when a MVD holds.

Generally, we may suppose that the A's and B's (left side and right side) of a MVD are disjoint. However, as with FD's, it is allowable to add some of the A's to the right side if we wish. Also note that unlike FD's, where we started with single attributes on the right and allowed sets of attributes on the right as a shorthand, with MVD's, we must  consider sets of attributes on the right instantly. As we shall see in Example (a) of "Reasoning About Multivalued Dependencies", it is not always possible to break the right sides of MVD's into single attributes.

A multivalued dependency guarantees that v exists

Example :  In "Multivalued Dependencies" example, we encountered a MVD that in our notation is expressed:

That is, for each star's name, the set of addresses appears in conjunction with each of the stars movies. For an example of how the formal definition of this MVD applies, consider the first and fourth tuples from "Multivalued Dependencies" figure:

If we let the first tuple be t and the second be u, then the MVD states that we must also find in R the tuple that has name C. Fisher, a street and city that agree with the first  tuple, and other attributes ( title and year) that agree with the second tuple. There is in fact such a tuple; it is the third tuple of "Multivalued Dependencies" figure.

Likewise, we could let t be the second tuple above and u be the first. Then the MVD tells us that there is a tuple of R that agrees with the second in attributes name, street, and city and with the first in name, title, and year. This tuple also exists; it is the second tuple of "Multivalued Dependencies" figure.