ITPEC FE Morning April 2018 Question 27

Topic: Databases

There is a “Delivery” table that has six (6) records. Which of the following is the functional dependency that is satisfied by this table? Here, “X → Y” indicates that X functionally determines Y.

Delivery

Delivery_dateDepartment_IDDepartment_nameDelivery_destinationComponent_IDQuantity
2016-08-21300Production department 2Chicago office1342300
2016-08-21300Production department 2Chicago office1342300
2016-08-25400Production department 1Boston factory2346300
2016-08-25400Production department 1Boston factory23461,000
2016-08-30500Research and development departmentBoston factory234630
2016-08-30500Research and development departmentNew York office134230

Explanation

Delivery_destination → Component_ID — it is the only option where each value on the left always comes with one value on the right.

X → Y holds when rows that share an X value always share the same Y value, so one counterexample is enough to disprove it.

  • •Delivery_destination: Chicago office always has 1342, Boston factory always has 2346 (three rows), New York office has 1342. No conflict, so the dependency holds.

Answer (b)

Why not others:

  • •(a) Delivery_date → Component_ID — 2016-08-30 appears with both 2346 and 1342
  • •(c) Department_ID → Component_ID — department 500 appears with both 2346 and 1342
  • •(d) Department_name → Delivery_destination — the Research and development department delivers to both Boston factory and New York office

Key rule: To test X → Y, group the rows by X and look for a group with two different Y values; a single such group disproves the dependency.

Databases · AI-written, may contain errors

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