125. What is Multivalued dependency?

Multivalued dependency denoted by X -> Y specified on relation schema R, where X and Y are both subsets of R, specifies the following constraint on any relation r of R: if two tuples t1 and t2 exist in r such that t1[X] = t2[X] then t3 and t4 should also exist in r with the following properties
> t3[x] = t4[X] = t1[X] = t2[X]
> t3[Y] = t1[Y] and t4[Y] = t2[Y]
> t3[Z] = t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]