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In our dbms class, these were the four properties of a primary key discussed for Oracle.

  • Unique
  • Not NULL
  • Fully functional dependency
  • Indexed

I understand all the properties except the 3rd one (fully functional dependency). Doesn't unique property ensure this?

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Perhaps you were taught wrong. Those four conditions are neither necessary nor sufficient to make a key. Keys must be irreducibly unique (a minimal superkey) and non-nullable. Non-partial dependence is a requirement of 2nd Normal Form but not a fundamental requirement of keys. Indexing also has nothing to do with whether a set of attributes qualify as a key or not. –  sqlvogel Dec 4 '12 at 21:29

1 Answer 1

up vote 4 down vote accepted

No, from uniqueness you cannot deduce full functional dependency.

I you have a relation (=table) with a candidate key(~primary key) P and additional attributes A,B,... than all attributes depend from P because P is the candidate key, but all atrributes depend from the combined superkey PA too (this means the key containing both P and A), but this is not a full functional dependency because they depend on a subset of attributes of PA, on the attribute P. PA is unique and not null but PA is no candidate key.

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