Solution:Step 1: Understand window frame defaultBy default, the window frame for ORDER BY is RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW.Step 2: Effect on LAST_VALUEThis means LAST_VALUE returns the value of the current row, not the last row in the partition.Step 3: Correct usageTo get the act

[Solved] Consider this query:SELECT emp_id, LAST_VALUE(salary) OVER (ORDER BY emp_id) FROM employees;Why might this query not return the expected last salary value for each employee? | PostgreSQL

PostgreSQL - Window Functions in PostgreSQL

Consider this query:

SELECT emp_id, LAST_VALUE(salary) OVER (ORDER BY emp_id) FROM employees;

Why might this query not return the expected last salary value for each employee?

ABecause the query lacks a PARTITION BY clause

BBecause the default window frame ends at the current row, so LAST_VALUE returns the current row's salary

CBecause LAST_VALUE requires a GROUP BY clause to work correctly

DBecause salary is not indexed

Step-by-Step Solution

Solution:

Step 1: Understand window frame default
By default, the window frame for ORDER BY is RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW.
Step 2: Effect on LAST_VALUE
This means LAST_VALUE returns the value of the current row, not the last row in the partition.
Step 3: Correct usage
To get the actual last value, specify the frame as ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING.
Final Answer:
Because the default window frame ends at the current row, so LAST_VALUE returns the current row's salary -> Option B
Quick Check:
Check window frame boundaries for LAST_VALUE [OK]

Quick Trick: LAST_VALUE needs full frame to get true last value [OK]

Common Mistakes:

Assuming LAST_VALUE returns last row without frame adjustment
Ignoring window frame clauses in window functions

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Consider this query:

Step 1: Understand window frame default

Step 2: Effect on LAST_VALUE

Step 3: Correct usage

Final Answer:

Quick Check:

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