Functional Dependencies (Cont.)

---

From Table Content to Functional Depencencies
Normally we determine FDs from understanding the data items and rules of enterprise (for example, each employee has a single phone number, employees can share a phone, etc.).

FDs are found from the content of the three tables, respectively. Again, the example is only to show what the FDs are, but is unusual.

Functional Dependency on Sets of Attributes
Given a table T with two sets of attributes X = A1 A2 ... Ak and Y = B1 B2 ... Bm, X→Y or X functionally determines Y if

• Two rows in T cannot agree in value on the attributes of X and simultaneously disagree in value on the attributes of Y, or


• Given any two rows r1 and r2 in a table T if r1(Ai)=r2(Ai) for every Ai in X, then r1(Bj)=r2(Bj) for every Bj in Y.

Finding Functional Dependencies
The database designer studies the data items and rules of enterprise, and figure out what the FDs should be.

For example, one set of the FDs of the data items given on the right is listed below:

emp_id   emp_name   emp_phone dept_name   dept_phone   dept_mgrname skill_id   skill_name   skill_date   skill_lvl

1. emp_id → emp_name  emp_phone  dept_name
2. dept_name → dept_phone  dept_mgrname
3. skill_id → skill_name
4. emp_id  skill_id → skill_date  skill_lvl