MediumCTEs

Departmental Headcount and Compensation Budget Cap

Frequently Asked By:GoogleMetaMicrosoft

Interactive In-Browser Workspace

Practice this problem on live schema tables in PostgreSQL 16 WASM with instant row verification and visual EXPLAIN plans.

Solve in Workbench

Problem Statement

Write a query using a CTE `dept_payroll` that computes total payroll and headcount per department from `dept_employees`. Join the CTE with `dept_budgets` on `dept_id`. Return: 1. `b.dept_name` 2. `p.headcount` 3. `p.total_payroll` 4. `b.budget_cap` 5. `budget_utilization_pct`: `ROUND(p.total_payroll * 100.0 / b.budget_cap, 1)` Order results by `budget_utilization_pct` descending, then by `b.dept_name` ascending.

Why Technical Interviewers Ask This

FP&A finance controllers monitor departmental payroll spend against annual operating budget allocations to flag departments nearing compensation caps.

Common Pitfalls & Interview Traps

Forgetting required sorting and deterministic tie-breaking.

Not accounting for NULL or zero-value denominators during calculations.

Using inefficient quadratic subqueries instead of single-pass ctes constructs.

Canonical Modern SQL Solution

PostgreSQL 16 / ANSI SQL

WITH dept_payroll AS (
        SELECT 
          dept_id,
          COUNT(*) AS headcount,
          SUM(salary) AS total_payroll
        FROM dept_employees
        GROUP BY dept_id
      )
      SELECT 
        b.dept_name,
        p.headcount,
        p.total_payroll,
        b.budget_cap,
        ROUND(p.total_payroll * 100.0 / b.budget_cap, 1) AS budget_utilization_pct
      FROM dept_payroll p
      JOIN dept_budgets b ON p.dept_id = b.dept_id
      ORDER BY budget_utilization_pct DESC, b.dept_name ASC;

Approach Breakdown

This query addresses the problem requirements by filtering and structuring data in CTEs. It meets the target criteria: Departmental payroll utilization percentages compared against budget caps.

Edge Cases to Verbally Clarify with Your Interviewer

The dataset contains zero matching records (must return valid empty result without crashing).

Multiple records tie on sorting criteria (must evaluate tie-breaking order deterministically).

Foreign keys or secondary attributes have NULL values.

Frequently Asked Follow-Up Questions

What makes this question a favorite among Google and Meta interviewers?

Interviewers use this problem to test whether candidates write defensive, scalable CTEs queries that run efficiently on multi-million row production databases without excessive memory usage.

How does the query optimizer execute this query?

Using single-pass table scans with index lookups or sort buffers. Running EXPLAIN (ANALYZE, BUFFERS) in the DB Exercises workbench visualizes the execution nodes and buffer hit cache.