SQL-Engineering-Handbook

# Module 18 · SQL Interview Questions **A structured SQL assessment system: 92 questions across 10 competency levels, built for candidates, interviewers and hiring managers.** [![Questions](https://img.shields.io/badge/questions-92-blue?style=for-the-badge)](#-current-coverage) [![Levels](https://img.shields.io/badge/levels-10%20(0%E2%80%939)-orange?style=for-the-badge)](#-competency-levels) [![Target](https://img.shields.io/badge/target-~490-lightgrey?style=for-the-badge)](#-current-coverage) [![License](https://img.shields.io/badge/license-MIT-brightgreen?style=for-the-badge)](#-contributing) [Taxonomy](/SQL-Engineering-Handbook/18_SQL_INTERVIEW_QUESTIONS/TAXONOMY_AND_FORMAT.html) · [Question Index](/SQL-Engineering-Handbook/18_SQL_INTERVIEW_QUESTIONS/QUESTION_INDEX.html) · [Practice Tracks](/SQL-Engineering-Handbook/18_SQL_INTERVIEW_QUESTIONS/PRACTICE_TRACKS.html) · [Interviewer Guide](/SQL-Engineering-Handbook/18_SQL_INTERVIEW_QUESTIONS/INTERVIEWER_GUIDE.html) · [Scoring Rubric](/SQL-Engineering-Handbook/18_SQL_INTERVIEW_QUESTIONS/SCORING_RUBRIC.html) · [Handbook Home](/SQL-Engineering-Handbook/)

🎯 What This Module Is

A structured SQL assessment system, not a list of trivia. It serves two audiences at once: people preparing for SQL interviews and people running them. Every question tests whether someone can apply SQL engineering judgment, not whether they’ve memorized syntax.

[!IMPORTANT] This module does not re-teach SQL. It assumes, and cross-links to, the handbook’s core curriculum (01_Fundamentals through 17_TRANSACTIONS_AND_CONCURRENCY). If a question’s underlying concept is unfamiliar, follow its Related Modules link and learn it there first.

👥 Who Should Use This

You are a…	Start here	What you get
🧑‍💻 Candidate	`PRACTICE_TRACKS.md`	Pre-built question sequences by role and length
🎤 Interviewer	`INTERVIEWER_GUIDE.md`	Interview methodology, not just a question list
🧭 Hiring manager	`SCORING_RUBRIC.md`	An eight-dimension competency scoring model
🛠️ Engineer self-assessing	`FAQ.md`	Guidance on honest self-scoring

flowchart LR
    T["TAXONOMY_AND_FORMAT<br/>learn the schema"] --> I["QUESTION_INDEX<br/>filter by level, difficulty, type"]
    I --> P["PRACTICE_TRACKS<br/>candidates"]
    I --> G["INTERVIEWER_GUIDE<br/>interviewers"]
    G --> S["SCORING_RUBRIC<br/>hiring managers"]
    P --> C["INTERVIEW_CHEATSHEET<br/>final recall"]

🪜 Competency Levels

Ten levels, numbered 0–9. Each level assumes the ones below it. Every level folder contains a QUESTION_BANK.md.

Level	Name	Question bank
0	Foundations	`01_FOUNDATIONS`
1	Core SQL	`02_CORE_SQL`
2	Analytical SQL	`03_ANALYTICAL_SQL`
3	Advanced SQL	`04_ADVANCED_SQL`
4	Database Engineering	`05_DATABASE_ENGINEERING`
5	Real-World SQL	`06_REAL_WORLD_SQL`
6	Production Debugging	`07_PRODUCTION_DEBUGGING`
7	Senior / Staff Engineering	`08_SENIOR_ENGINEERING`
8	Interviewer / Hiring	`09_INTERVIEWER_QUESTIONS`
9	SQL System Design	`10_SQL_SYSTEM_DESIGN`

_{All 10 levels (L0–L9), all built}

📊 Current Coverage

92 questions, built across all 10 competency levels. This is the third expansion pass beyond the original 48-question breadth pilot, targeted at Levels 1–2 (the largest remaining gap to the ~490 target). No new levels were added.

Level	Name	Built	Target	Progress
0	Foundations	10	~40	`▰▰▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 25%
1	Core SQL	16	~60	`▰▰▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 27%
2	Analytical SQL	17	~80	`▰▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 21%
3	Advanced SQL	9	~70	`▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 13%
4	Database Engineering	8	~70	`▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 11%
5	Real-World SQL	8	~70	`▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 11%
6	Production Debugging	6	~40	`▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 15%
7	Senior / Staff Engineering	6	~40	`▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 15%
8	Interviewer / Hiring	6	~30	`▰▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 20%
9	SQL System Design	6	~30	`▰▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 20%
	Total	92	~490	`▰▰▰▰▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱▱` 19%

What each question contains

Optional fields are omitted where they would add little value, not padded in for uniformity.

Element	Coverage
`### Explanation` or `### Expected Answer`	66 of 92 questions
`### Interviewer Follow-Up`	86 of 92 questions
Explicit `### Common Mistake` (or Level 8’s `Weak Answer`)	About 10 of 92 carry the literal heading
Named “senior-level extension”	A handful. Levels 7–9 serve as the extension by design.
Explicit `Related Modules` link	26 of 92 questions

Format notes and known gaps (read before auditing)

- Scenario-style and interviewer-facing questions (for example Levels 6 and 8) use their own section structure instead of the exact headings above. - **Q041–Q044, Q077 and Q078** have no separate follow-up section. - On most questions the common-mistake framing is folded into the explanation prose instead of being broken out as its own section. **Retrofitting a dedicated section onto every question is a real coverage gap, not yet done.** It is tracked in [`AUDIT_REPORT.md`](/SQL-Engineering-Handbook/18_SQL_INTERVIEW_QUESTIONS/AUDIT_REPORT.html) rather than silently claimed here. - The Level 7–9 questions act as the senior-level extension (harder variants of the same skills), rather than each L0–L6 question carrying its own bolt-on senior section. See also [`FINAL_VALIDATION_REPORT.md`](/SQL-Engineering-Handbook/18_SQL_INTERVIEW_QUESTIONS/FINAL_VALIDATION_REPORT.html).

🧩 How the Question Bank Is Organized

Full schema: TAXONOMY_AND_FORMAT.md. In short:

Dimension	Values
Competency levels	10, numbered 0–9, each assuming the ones below it
Difficulty ratings	5, assigned by objective criteria: 🟢 🟡 🟠 🔴 ⚫
Question types	10: Write SQL · Predict Output · Debug · Optimize · Explain · Compare · Data Reasoning · Execution Plan · Design · Production Incident

Use QUESTION_INDEX.md to filter every question by level, difficulty and type.

File	Purpose
`TAXONOMY_AND_FORMAT.md`	The schema. Read this first.
`QUESTION_INDEX.md`	Every question, filterable by level, difficulty and type
`INTERVIEWER_GUIDE.md`	How to actually run an interview with this bank
`SCORING_RUBRIC.md`	Eight-dimension competency scoring model
`INTERVIEW_CHEATSHEET.md`	Compact recall reference
`PRACTICE_TRACKS.md`	Pre-built question sequences by role and length
`GLOSSARY.md`	Terms used throughout, linked to full explanations
`FAQ.md`	Practical usage questions
`AUDIT_REPORT.md`	Tracked coverage gaps and audit findings
`FINAL_VALIDATION_REPORT.md`	Validation results
`scripts/validate_questions.py`	Structural validation script

🖼️ Diagram Gallery

Competency pyramid

_{All 10 levels (L0–L9), all built · source}

Interview flow

_{Assessment workflow, track selection to scoring · source}

🗃️ Dataset Approach

Questions reuse the handbook’s shared employes / departments / locations schema wherever the scenario allows.
Level 5 (Real-World) and Level 9 (System Design) questions define small inline schemas for their specific domain (e-commerce, fintech, SaaS, healthcare, logistics) rather than introducing a large parallel dataset.

About a third of questions (26 of 92) carry an explicit Related Modules link to the one curriculum lesson they draw on. The rest lack a clean single link, not a connection:

Window-function mechanics in Level 2 are mostly self-contained.
Scenario-driven Level 5, 8 and 9 questions typically span several lessons at once.
Level 4+ leans heavily on 16_QUERY_OPTIMIZATION and 17_TRANSACTIONS_AND_CONCURRENCY, even without a per-question link. This module tests whether you can apply what those teach.

🗂️ Repository Layout

18_SQL_INTERVIEW_QUESTIONS/
├── README.md
├── TAXONOMY_AND_FORMAT.md
├── QUESTION_INDEX.md
├── INTERVIEWER_GUIDE.md
├── SCORING_RUBRIC.md
├── INTERVIEW_CHEATSHEET.md
├── PRACTICE_TRACKS.md
├── GLOSSARY.md
├── FAQ.md
├── AUDIT_REPORT.md
├── FINAL_VALIDATION_REPORT.md
├── 01_FOUNDATIONS/            └─ QUESTION_BANK.md
├── 02_CORE_SQL/               └─ QUESTION_BANK.md
├── 03_ANALYTICAL_SQL/         └─ QUESTION_BANK.md
├── 04_ADVANCED_SQL/           └─ QUESTION_BANK.md
├── 05_DATABASE_ENGINEERING/   └─ QUESTION_BANK.md
├── 06_REAL_WORLD_SQL/         └─ QUESTION_BANK.md
├── 07_PRODUCTION_DEBUGGING/   └─ QUESTION_BANK.md
├── 08_SENIOR_ENGINEERING/     └─ QUESTION_BANK.md
├── 09_INTERVIEWER_QUESTIONS/  └─ QUESTION_BANK.md
├── 10_SQL_SYSTEM_DESIGN/      └─ QUESTION_BANK.md
├── scripts/
│   └── validate_questions.py
└── assets/
    ├── images/hero.svg
    └── diagrams/
        ├── competency-pyramid.svg
        └── interview-flow.svg

✅ Validation

Run from this module’s root:

python3 scripts/validate_questions.py .

It checks duplicate IDs, missing required fields and broken relative links. See the script’s docstring for documented limitations.

🤝 Contributing

Follow the format and deduplication policy in TAXONOMY_AND_FORMAT.md.
Assign the next sequential Q### ID.
Run scripts/validate_questions.py before submitting.

Released under the MIT license.

[⬅️ **Previous:** 17 — Transactions & Concurrency](/SQL-Engineering-Handbook/17_TRANSACTIONS_AND_CONCURRENCY/) | [🏠 **SQL Engineering Handbook**](/SQL-Engineering-Handbook/) _{Part of the SQL Engineering Handbook · Module 18 of the series}

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SQL-Engineering-Handbook

📑 Table of Contents

🎯 What This Module Is

👥 Who Should Use This

🪜 Competency Levels

📊 Current Coverage

What each question contains

🧩 How the Question Bank Is Organized

🧭 Navigation

🖼️ Diagram Gallery

🗃️ Dataset Approach

🗂️ Repository Layout

✅ Validation

🤝 Contributing

🧭 Module Navigation

SQL-Engineering-Handbook

📑 Table of Contents

🎯 What This Module Is

👥 Who Should Use This

🪜 Competency Levels

📊 Current Coverage

What each question contains

🧩 How the Question Bank Is Organized

🧭 Navigation

🖼️ Diagram Gallery

🗃️ Dataset Approach

🔗 Related Modules

🗂️ Repository Layout

✅ Validation

🤝 Contributing

🧭 Module Navigation