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Grade 11 · lesson 2 of 4

Check Key Uniqueness Before Joining

Find duplicate identifiers and stop an ambiguous lookup.

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Supplied materials

  • Paper and pencil; an adult may read aloud. No device, account or purchase is needed.

Learn the idea

A lookup expects a key to identify one row. If a supposedly unique key appears twice with different values, a key match alone no longer chooses one record. The correct response depends on the data contract; here the contract requires every object_id to be unique and any duplicate to be rejected before joining.

Supplied objects table: O1 = map; O2 = compass; O2 = globe. A request references object_id O2. Both compass and globe match, so choosing the first row would hide the violation. Trace a validation algorithm: set seen to empty; for each key, if it is already in seen report duplicate, otherwise add it.

  1. Write the supplied three rows with their identifiers.
  2. Track seen after each key: {O1}, then {O1,O2}.
  3. On the last O2, report duplicate and stop the lookup under this contract.
  4. Repair with a verified invented identifier O3 for globe, then retest uniqueness and the O2 request.
Worked example

See the method in action

Validation sees O1 new, O2 new, O2 repeated: duplicate O2. After the exercise repair to O3 = globe, seen is {O1,O2,O3}; request O2 unambiguously returns compass. A real data repair would require confirming the intended identifiers, not guessing.

Your turn

Check your reasoning

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Table keys are K1, K2, K1. Which key violates uniqueness?

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See the answer and explanation

K1

K1 occurs twice. The uniqueness requirement concerns the key field; other non-key values can legitimately repeat.

Take it further

Change the repaired table’s globe key back to O1. Expected validation error: duplicate O1. The specific conflicting key changes, but validation still rejects the table.

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These LifeStarter passages, scenarios and exercises are authored for this Academy with AI assistance and review. External resources keep their publishers’ terms. A grade label suggests a starting level; it does not establish alignment with your school’s complete requirements.