SQL 17 : Problem Solving Template (Incremental Build)

D

Qubits of DPK

April 11, 2026

Core DBMS
Use this format every time you solve a SQL problem. Start simple. Build up. Never jump to the final query.

1. Problem Understanding

  • What is the expected output?
  • What columns should appear?
  • What filters or conditions exist?

2. Tables Involved

List tables and the key columns you'll use.

3. Base Query — Start Simple

sql
QUBITS OF DPK
1SELECT *
2FROM employees;
Why this table? State it.

4. Add JOINs (if needed)

sql
QUBITS OF DPK
1SELECT *
2FROM employees e
3JOIN departments d ON e.department_id = d.department_id;
Why the JOIN? What does it unlock?

5. Add Filters

sql
QUBITS OF DPK
1WHERE d.location = 'Seattle';
State the filtering rule in plain English.

6. Add Aggregation (if needed)

sql
QUBITS OF DPK
1SELECT department_id, AVG(salary) AS avg_salary
2FROM employees
3GROUP BY department_id;
What's being aggregated? Why grouping?

7. Add Window Functions (if needed)

sql
QUBITS OF DPK
1AVG(salary) OVER (PARTITION BY department_id)
How does this differ from GROUP BY? (GROUP BY collapses rows. Window functions keep all rows and add a computed column.)

8. Add Derived Calculations

sql
QUBITS OF DPK
1SUM(amount) * 100.0 / SUM(SUM(amount)) OVER ()
Break the formula down step by step.

9. Formatting / Ranking / Limiting

sql
QUBITS OF DPK
1ORDER BY avg_salary DESC
2LIMIT 5;
3-- or
4RANK() OVER (ORDER BY avg_salary DESC)

10. Final Query

Write the complete assembled query.

11. Concepts Used

List: JOIN, GROUP BY, Window Function, Subquery, EXISTS, etc.

12. Common Mistakes (Interview Focus)

Document what could go wrong here.

️ Common Interview Mistakes