DBMS

A **Closure** is a set of FDs is a set of all possible FDs that can be derived from a given set of FDs. It is also referred as a** Complete **set of FDs. If F is used to donate the set of FDs for relation R, then a closure of a set of FDs implied by F is denoted by F^{+}. Let's consider the set F of functional dependencies given below:

**F = {A -> B, B -> C, C -> D}**

from F, it is possible to derive following dependencies.

A -> A ...By using Rule-4, Self-Determination.

A -> B ...Already given in F.

A -> C ...By using rule-3, Transitivity.

A -> D ...By using rule-3, Transitivity.

Now, by applyiing Rule-6 Union, it is possible to derive A^{+} -> ABCD and it can be denoted using A -> ABCD. All such type of FDs derived from each FD of F form a closure of F. Steps to determine F^{+}example:

- Determine each set of attributes
**X**that appears as a left hand side of some FD in F. - Determine the set
**X**of all attributes that are dependent on^{+}**X**, as given in above example. - In other words,
**X**represents a set of attributes that are functionally determined by X based on F. And, X^{+}^{+}is called the**Closure of X under F.** - All such sets of
**X**, in combine, Form a closure of F.^{+}

**Input : **Let F be a set of FDs for relation R.
**Steps:**
1.** X**^{+} = X //initialize X^{+ }to X
2. For each FD : Y -> Z in F Do
If **Y **⊆ **X**^{+ }Then //If Y is contained in X^{+}
**X**^{+ } = X^{+ } ∪ Z //add Z to X^{+}
End If
End For
3. Return **X**^{+ }//Return closure of X
**Output :** Closure **X**^{+ }of **X** under **F**

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