A key is a special case of a FD
Remember the definition of a FD:
A functional dependency X → Y holds
in relation R if for two different rows, r, s:
r[X] ≠ s[X]
or r[Y] = s[Y].
Key
- A key uniquely identifies a row in a table.
- I.e., For two different rows, r and s: r[key] != s[key].
- The condition of an FD is satisfied.
- So we have a FD from the key to every non-key column of the table.
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