Functional Dependencies

  1. An attribute is functionally dependent if its value is determined by another attribute.
  2. That is, if we know the value of one (or several) data items, then we can find the value of another (or several).
  3. Functional dependencies are expressed as Xrarrow.gif (66 bytes)Y, where X is the determinant and Y is the functionally dependent attribute.
  4. That is, Xrarrow.gif (66 bytes)Y means "if I know the value of X, then I can look up the value of Y" (in a table or somewhere).
  5. If Ararrow.gif (66 bytes)(B,C) then Ararrow.gif (66 bytes)B and Ararrow.gif (66 bytes)C.
  6. If (A,B)rarrow.gif (66 bytes)C, then it is not necessarily true that Ararrow.gif (66 bytes)C and Brarrow.gif (66 bytes)C.
  7. If Xrarrow.gif (66 bytes)Y and Yrarrow.gif (66 bytes)X, then X and Y are in a 1-1 relationship.
  8. Bottom line: Xrarrow.gif (66 bytes)Y is like a mathematical function Y=f(X) since for each value of X there must be a unique value of Y, e.g. y=x+x.