DA0-002 Data Acquisition and Preparation Practice Question
An analyst writes a SQL query that uses a window function: SELECT employee_id, salary, LAG(salary, 1) OVER (ORDER BY salary DESC) AS prev_salary FROM employees. What does the LAG function return for the row with the highest salary?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
NULL
LAG returns the previous row's value in the ordered partition. For the first row (highest salary), there is no previous row, so it returns NULL.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The same salary value
Why it's wrong here
LAG does not return the current value.
- ✓
NULL
Why this is correct
LAG returns NULL if there is no preceding row.
- ✗
The next highest salary
Why it's wrong here
That would be LEAD, not LAG.
- ✗
Zero
Why it's wrong here
LAG returns NULL by default, not zero.
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