Point72·Data Scientist·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Point72 Data Scientist interview with a SQL question centered on department-level aggregation. Pretty standard stuff but the LEFT JOIN detail matters more than it looks.

Questions Asked (1)

Write a SQL query that shows each department name alongside its student count, making sure departments with zero students still appear in the results.

Data ModelingTechnical Trade-offs

Author's notes

The zero-count departments are the whole point.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the schema and the definition of 'student count' (e.g., distinct students per department). Then write a LEFT JOIN from departments to students, grouping by department, and use COUNT on the student side to handle NULLs. Finally, discuss performance considerations and edge cases like departments with no students.

Pro tip: Mention that using COUNT(student_id) instead of COUNT(*) is crucial because COUNT(*) would count the NULL row from the LEFT JOIN, incorrectly showing 1 for empty departments. This shows attention to detail and understanding of SQL semantics.

1. Clarify requirements and schema

Ask about the table structures (e.g., departments, students) and confirm that 'student count' means the number of students per department, including zero for departments with no students.

2. Choose the right join type

Use a LEFT JOIN from departments to students to ensure all departments appear, even those without matching students.

3. Aggregate correctly

Group by department name (and ID if needed) and use COUNT on a student column (e.g., student_id) to count only non-NULL values, yielding 0 for departments with no students.

4. Write and explain the query

Present the SQL query clearly, explaining each clause. Optionally, discuss alternative approaches like using a subquery or window function.

5. Discuss performance and edge cases

Mention indexing on foreign keys, potential performance issues with large datasets, and how the query handles duplicate student records or NULL department IDs.

Key Points to Mention

LEFT JOIN ensures all departments are included, even with zero students.
COUNT(student_id) ignores NULLs, so empty departments get 0, unlike COUNT(*).
Grouping by department name (and possibly department ID) is necessary for aggregation.
Indexes on the join key (e.g., department_id in students) can improve performance.
Consider if 'student count' means distinct students; use COUNT(DISTINCT student_id) if duplicates exist.
Alternative: Use a correlated subquery or a derived table, but LEFT JOIN is typically more efficient.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.