Examine the ER diagram below.

Which one of the statements about this diagram is invalid?

Question

Examine the ER diagram below.

Which one of the statements about this diagram is invalid?

BlackTom AI · Accepted Answer

Double vertical bar (| |) = exactly one (mandatory 1). Circle (o) = zero (optional). Crow’s foot = many. Read each relationship end from the entity’s side to determine minimum and maximum cardinalities.

1) Relevant knowledge points and concepts
- Cardinality notation: single vertical line = mandatory 1 (min = 1, max = 1), circle = optional min = 0, crow’s foot = many (max = N).
- For a binary relationship, examine the symbol placed at each entity’s end to determine that entity’s min..max participation.

2) Step‑by‑step reasoning from the diagram
- CUSTOMER —owns— CAR: At the CUSTOMER end there is a crow’s foot toward CAR and at the CAR end there is a single vertical line (no circle). That means each CAR has exactly one OWNER (min 1, max 1); a CUSTOMER can own many CARS. Therefore “Every car must have one owner” is true.
- CAR —gets— MAINTENANCE: The CAR end is marked with a circle (optional) toward MAINTENANCE, and MAINTENANCE has a crow’s foot on its side. This means a CAR may have zero or many MAINTENANCE records — so “Some cars do not get maintenance” is true.
- MAINT_LINE —is written in— PART: On the PART side there is a double vertical line (mandatory 1) and on the MAINT_LINE side there is a circle with a crow’s foot (optional many). Reading that, a PART is associated with exactly one MAINT_LINE (min 1, max 1) but a MAINT_LINE can be associated with zero or many PARTs (min 0, max N).

3) Intermediate interpretation (explicit)
- MAINT_LINE min cardinality to PART = 0 (because of the circle), max = many (crow’s foot). So a maintenance line can have zero parts attached.

4) Why the correct answer is correct
- The statement “A maintenance line corresponds to at least one spare part” asserts min ≥ 1 for MAINT_LINE → PART. The diagram shows a circle at the MAINT_LINE end, indicating min = 0. Therefore that statement is false (invalid).

5) Why the other options are incorrect (i.e., why they are valid)
- “Every car must have one owner”: diagram enforces mandatory single owner for CAR → valid.
- “Not every spare part is used for maintenance”: PART participation allows some parts not linked to maintenance (optional on the MAINT_LINE side) → valid.
- “Some cars do not get maintenance”: CAR side is optional toward MAINTENANCE (circle) → valid.

6) Conclusion
- The invalid statement is “A maintenance line corresponds to at least one spare part,” which matches the provided answer.

Examine the ER diagram below. Which one of the statements about this diagram is invalid?单项选择题

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