What does the following query do?

Question

BlackTom AI · Accepted Answer

ROWNUM is a pseudo-column in Oracle that numbers rows as they are returned; wrapping an ORDER BY inside a subquery ensures sorting happens before the outer ROWNUM filter is applied. Therefore this query returns the top five rows from employees sorted by hire_date descending — the five most recently hired employees.

1) Relevant knowledge points and concepts
- ORDER BY sorts result rows; DESC sorts from newest/largest to oldest/smallest.
- ROWNUM is assigned to rows in the order they are produced by a query. If you want the top N after ordering, you must order in a subquery and then apply ROWNUM in the outer query.
- Using ORDER BY in the inner query and WHERE ROWNUM <= N in the outer query is the canonical pattern in Oracle for “top N” after sorting.

2) Step-by-step reasoning
- Inner query: SELECT * FROM employees ORDER BY hire_date DESC
  - Produces the full employees result set sorted with the newest hire_date first.
- Outer query: SELECT * FROM (inner_query) WHERE ROWNUM <= 5;
  - The outer query reads the ordered rows, assigns ROWNUM starting at 1, and keeps rows where ROWNUM is 1..5.
- Result: the first five rows from the descending-hire_date ordered list.

3) Intermediate steps (conceptual)
- Sorted list example (hire_date newest → oldest): E1, E2, E3, E4, E5, E6, ...
- Outer filter applies ROWNUM: keep E1 (ROWNUM=1) … E5 (ROWNUM=5), discard the rest.

4) Why the correct answer is correct
- Because ORDER BY DESC puts most recent hires first and the outer ROWNUM <= 5 takes exactly the first five of that ordered sequence, the query returns the five most recently hired employees.

5) Why other options are incorrect
- “Returns 5 random employees”: incorrect — ORDER BY DESC deterministically sorts by hire_date.
- “Returns employees with the 5 earliest hire dates”: incorrect — earliest would require ORDER BY hire_date ASC (or ORDER BY hire_date without DESC).
- “Throws an error due to invalid ROWNUM usage with ORDER BY”: incorrect — using ORDER BY inside a subquery and ROWNUM outside is valid and the intended pattern.

Conclusion: The query returns the 5 most recently hired employees, matching the provided answer.

What does the following query do?单项选择题

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