ITPEC FE Morning October 2022 Question 26
In a relational database, which of the following is an appropriate purpose for defining a foreign key?
Explanation
A constraint that keeps references between related tables consistent — a foreign key enforces referential integrity.
A foreign key is a column, or set of columns, whose values must match the primary key of another table or be NULL. With orders. referencing customers., the DBMS rejects an order for a customer code that does not exist, and rejects (or cascades, if so defined) deleting a customer who still has orders. No row can point at something that is not there.
Answer (b)
Why not others:
- •(a) — fragmentation from repeated deletes and inserts is a physical storage problem, fixed by reorganising the storage area
- •(c) — restoring damaged records is recovery, done from backups and the log (roll forward), not from relationships between tables
- •(d) — storing related rows physically close together is clustering, a performance measure rather than a constraint
Key rule: Keys and constraints protect the logical correctness of the data; storage layout, speed and failure recovery are handled by separate DBMS mechanisms.
Databases · AI-written, may contain errors
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