What is meant by functional dependencies, Database Management System

What is meant by functional dependencies?

Consider a relation schema R and α C R and β C R. The functional dependency α ->β  holds on relational schema R if in any legal relation r(R), for all pairs of tuples t1 and t2 in r like t1 [α] =t2 [α], and also t1 [β] =t2 [β].

 

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