30-Day-SQL-Challenge

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30-Day-SQL-Challenge

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Practice SQL for 30 days with Stephen

30-Day-SQL-Challenge 최신버젼 다운로드

최종 버전 다운로드 (.zip)

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By Day 30, you'll be the SQL person on your team. 
 The one others ask. The one who reads queries fast. The one who debugs the reports nobody else can fix. 
 This is the 30-day structure that gets you there - daily lessons, real datasets, real exercises.

20 OF 30 &nbsp;&middot;&nbsp; tap any tile to start watching

Tap any unlit tile to see what is coming.

---

## How it works

Five steps. Every day. Thirty days.

```mermaid
flowchart LR
 A([Fork the repo]) --> B([Watch the day's video])
 B --> C([Run the SQL in pgAdmin])
 C --> D([Solve the exercises])
 D --> E([Check your work against solutions])

style A fill:#312e81,stroke:#a5b4fc,stroke-width:2px,color:#ffffff
    style B fill:#4338ca,stroke:#a5b4fc,stroke-width:2px,color:#ffffff
    style C fill:#4f46e5,stroke:#a5b4fc,stroke-width:2px,color:#ffffff
    style D fill:#6366f1,stroke:#a5b4fc,stroke-width:2px,color:#ffffff
    style E fill:#f59e0b,stroke:#fbbf24,stroke-width:2px,color:#1f2937
```

Each day gives you:
- A **video lesson** walking through the concept with real examples
- A **dataset** you set up yourself in pgAdmin - you build the tables, not just read about them
- **Exercises** that drop you into a real role solving a real problem
- **Solutions** so you can check your work and see how you did

### Getting the files (new to GitHub?)

Every day's `setup.sql`, `exercise.sql`, and `solutions.sql` live in that day's folder (for example [`day-01/`](day-01/)). If the repo page looks like "just a list of folders", that is the right place - open a `day-XX` folder to see its files. To get them onto your machine, pick one:

- **Download (easiest):** click the green **Code** button near the top of this page, then **Download ZIP**. Unzip it and open the `day-XX` folder for the day you are on.
- **Clone (if you have Git):** run `git clone https://github.com/sdw-online/30-Day-SQL-Challenge.git` in your terminal.
- **Read one file online:** click into any `day-XX` folder, click the `.sql` file, then click **Raw** to copy the contents straight into pgAdmin.

Then open the day's `setup.sql` in pgAdmin and run it to create the tables before you start the exercises.

---

## Why This Challenge?

**You don't get good at SQL by watching someone else write it.** You get good by writing it yourself, every day, with data you can actually see and query. That's what this challenge is built around - not theory, not slides, just you and a database.

**30 minutes a day beats a weekend bootcamp.** It's tempting to binge-watch a 12-hour course on a Saturday and call it done. But that's exposure, not skill. When you write SQL every single day for 30 days, it stops being something you "learnt once" and becomes something you just know. The consistency is the point.

**Every exercise puts you in a real job.** You're not answering textbook questions here. You're a data analyst at a regional education authority. You're an analytics engineer at a logistics company. You're debugging a fintech lending pipeline. The scenarios are modelled on real work - logistics, healthcare, finance, infrastructure - so when you do land that role, nothing feels unfamiliar.

---

## "But can't AI write SQL for me?"

Sure, AI can generate SQL. But here's what it can't do: guarantee the output is correct, performant, or even answering the right question. And when it gets it wrong - which it regularly does - someone needs to spot the mistake, understand why it happened, and fix it. That someone needs SQL fluency.

But honestly, the AI argument misses the bigger picture. SQL isn't just a language you type into a database. It's a way of thinking:

- **It teaches you to decompose problems.** Every complex query starts as a vague question like "which schools are underperforming?" You learn to break that down - what does underperforming mean? Compared to what? Over what time period? SQL forces you to be specific, and that skill transfers to everything you do with data.
- **It teaches you to be precise.** A database won't guess what you meant. If your logic is wrong, your results are wrong - silently. You learn to think about edge cases, NULLs, duplicates, and assumptions. That kind of rigour makes you a better analyst, engineer, and thinker.
- **It gives you direct access to the truth.** When someone says "revenue is up 20%", you can check. When a dashboard looks wrong, you can query the source yourself. SQL removes the middleman between you and the data - and that's powerful in any role.

So the goal of this challenge isn't to memorise syntax. In the AI age, the syntax is the one part a machine can hand you on demand - and it usually will. The goal is **judgement**: knowing what to ask, reading a query and telling whether it's actually right, and catching the AI when it's confidently wrong. Writing the syntax yourself as you go builds that judgement, which is exactly why it's worth doing - but memorising it was never the point.

AI makes SQL faster to write. This challenge makes you someone who knows what to write - and can tell when the AI has got it wrong.

---

## Where should I start?

Not everyone's starting from scratch - and that's fine. Pick the door that matches where you are.

```mermaid
flowchart TD
 Q{What's your SQL level?}

click D1 "https://github.com/sdw-online/30-Day-SQL-Challenge/tree/main/day-01" _blank
    click D8 "https://github.com/sdw-online/30-Day-SQL-Challenge/tree/main/day-08" _blank
    click D15 "https://github.com/sdw-online/30-Day-SQL-Challenge/tree/main/day-15" _blank
    click D22 "https://github.com/sdw-online/30-Day-SQL-Challenge/tree/main/day-22" _blank

classDef start fill:#1e1b4b,stroke:#a5b4fc,stroke-width:2px,color:#ffffff
    classDef day fill:#4338ca,stroke:#818cf8,stroke-width:2px,color:#ffffff
    class Q start
    class D1,D8,D15,D22 day
```

Click any node to jump straight to that day's folder.

If diagrams don't render in your client, here's the short version:

- **Never written SQL?** → [Day 1](./day-01) - PostgreSQL setup + your first query
- **Basics rusty?** → [Day 8](./day-08) - Week 2 covers what most people forget first
- **Comfortable but no JOINs?** → [Day 15](./day-15) - where most people in data jobs realise they've been winging it
- **Already use SQL at work?** → [Day 22](./day-22) - window functions, MERGE, query optimisation, capstone

---

---

## Already know SQL? Start with the questions

If the syntax is not your problem and you just want **real questions to answer**, skip the
lessons. Every day below has a `questions.md`: the exercise written as a question to solve, with
what to return and a way to check yourself. **The technique is not named** - working out which
tool the question needs is most of the skill - and there are no solutions on the page.

| Day | Topic | Questions |
|---|---|:---:|
| [08](day-08/questions.md) | NULL handling | 4 |
| [09](day-09/questions.md) | String and numeric functions | 4 |
| [10](day-10/questions.md) | Date functions and CAST | 6 |
| [11](day-11/questions.md) | CASE WHEN | 5 |
| [12](day-12/questions.md) | Subqueries and temp tables | 4 |
| [13](day-13/questions.md) | CTEs | 4 |
| [15](day-15/questions.md) | JOINs part 1 | 4 |
| [16](day-16/questions.md) | CROSS and self joins | 5 |
| [17](day-17/questions.md) | UNION and UNION ALL | 4 |
| [18](day-18/questions.md) | Normalisation | 3 |
| [19](day-19/questions.md) | Recursive CTEs | 4 |
| [20](day-20/questions.md) | Star schema | 3 |

**50 questions across 12 days.** Run that day's `exercise.sql` first to create the tables, then
answer without opening `solutions.sql`. The remaining days are being written up the same way.

---

## Curriculum

| Day | Topic | What You'll Do | Video |
|:---:|-------|----------------|:-----:|
| 01 | Introduction to SQL & Databases | Set up PostgreSQL, create your first database, and run your first query Introduction to SQL & Databases - the thing most people get wrong. | [Watch](https://www.youtube.com/watch?v=mFIMPhiO-N0) |
| 02 | SELECT & WHERE | Pull specific data from a table and filter rows using conditions The single most-used SQL clause. Get it wrong, get nothing. | [Watch](https://www.youtube.com/watch?v=-0uVBtXCZ_s) |
| 03 | ORDER BY & LIMIT | Sort your results and control how many rows come back Sorting is easy. Sorting NULLs the right way is not. | [Watch](https://www.youtube.com/watch?v=s86nI9dPZqY) |
| 04 | Aggregate Functions & GROUP BY | Count, sum, and average your data - then group it to find patterns Aggregates have one trap that turns reports into lies. | [Watch](https://www.youtube.com/watch?v=7IWrvTIrIkg) |
| 05 | INSERT, UPDATE & DELETE | Add new rows, change existing ones, and remove what you don't need The day your habits decide whether prod survives the week. | [Watch](https://www.youtube.com/watch?v=NJ4ujmOZt60) |
| 06 | Primary & Foreign Keys | Understand how tables relate to each other and why constraints matter Primary & Foreign Keys - the thing most people get wrong. | [Watch](https://www.youtube.com/watch?v=1AdFU8Vdq-0) |
| 07 | **Project:** Freight & Logistics Report | Put it all together - build a performance report from real shipping data The day you stop learning and start building. | [Watch](https://youtu.be/fiBYAziNtGI) |

| Day | Topic | What You'll Do | Video |
|:---:|-------|----------------|:-----:|
| 08 | NULL Handling | Deal with missing data without breaking your queries NULL is not zero. Get this wrong and your aggregates lie. | [Watch](https://www.youtube.com/watch?v=0nH464EoZ9w) |
| 09 | String & Numeric Functions | Clean messy text, extract parts of strings, and round numbers properly Messy text is everywhere. The fix is one chapter away. | [Watch](https://www.youtube.com/watch?v=h6J7AajBD6w) |
| 10 | Date Functions & CAST | Work with dates, calculate time differences, and convert between types Dates have more traps than any other type. Spot them before they spot you. | [Watch](https://youtu.be/Iturx2kgs1A) |
| 11 | CASE WHEN | Add conditional logic to your queries - like if/else but inside SQL Most people overuse CASE WHEN. There is a cleaner way. | [Watch](https://youtu.be/eZ5iTTsKGkI) |
| 12 | Subqueries & Temp Tables | Nest queries inside each other and store intermediate results for reuse Subqueries & Temp Tables - the thing most people get wrong. | [Watch](https://youtu.be/SOt5jUrzKOU) |
| 13 | CTEs (Part 1) | Write cleaner, more readable queries using Common Table Expressions CTEs are why your seniors read SQL faster than you. Yet. | [Watch](https://youtu.be/IijQJAfqcJc) |
| 14 | **Project:** Fleet Intelligence Pipeline | Build a multi-step data pipeline using everything from Week 2 The day you stop learning and start building. | [Watch](https://youtu.be/afIJ4VsQYSo) |

| Day | Topic | What You'll Do | Video |
|:---:|-------|----------------|:-----:|
| 15 | JOINs Part 1: INNER, LEFT, RIGHT, FULL OUTER | Connect tables together and understand what each JOIN type keeps and drops JOINs look easy until they silently drop your data. | [Watch](https://youtu.be/wtBxs_iDLo4) |
| 16 | JOINs Part 2: CROSS & Self Joins | Generate combinations and compare rows within the same table JOINs look easy until they silently drop your data. | [Watch](https://youtu.be/ZYwPGw4ghkI) |
| 17 | UNION & UNION ALL | Stack result sets on top of each other and know when to deduplicate UNION & UNION ALL - the thing most people get wrong. |[Watch](https://youtu.be/wlohArgOSd4)|
| 18 | Normalisation & Denormalisation | Understand why databases split data across tables - and when to flatten it The rule that decides whether your schema scales or rots. |[Watch](https://youtu.be/dhdWwX8DAEg)|
| 19 | Recursive CTEs | Query hierarchical data like org charts and category trees CTEs are why your seniors read SQL faster than you. Yet. |[Watch](https://youtu.be/DI1swwiKxUc)|
| 20 | Data Modelling (Star Schema) | Design fact and dimension tables the way analytics teams actually do it How analytics teams actually structure data. Not how textbooks teach it. |[Watch](https://youtu.be/bZjfSWOIsBI)|
| 21 | **Project:** Recruitment Analytics | Model a messy applicant tracking system and find what actually predicts a hire The day you stop learning and start building. | Coming soon |

| Day | Topic | What You'll Do | Video |
|:---:|-------|----------------|:-----:|
| 22 | Window Functions Part 1 | Rank rows, number them, and calculate running totals without GROUP BY Window functions replace 80% of the SQL you used to write. | Coming soon |
| 23 | Window Functions Part 2 | Compare current rows to previous ones with LAG, LEAD, and QUALIFY Window functions replace 80% of the SQL you used to write. | Coming soon |
| 24 | SCD Types & MERGE | Track how data changes over time and upsert rows in one statement SCD Types & MERGE - the thing most people get wrong. | Coming soon |
| 25 | Views & Materialised Views | Save queries as reusable objects and pre-compute expensive results Views & Materialised Views - the thing most people get wrong. | Coming soon |
| 26 | Information Schema & Metadata | Query the database about itself - table sizes, column types, constraints Bad charts hide insight. Good ones force it. | Coming soon |
| 27 | CREATE FUNCTION (UDFs) | Build your own reusable SQL functions for logic you use repeatedly CREATE FUNCTION (UDFs) - the thing most people get wrong. | Coming soon |
| 28 | EXPLAIN & Indexing | Read query plans, spot bottlenecks, and make your queries faster Lookups separate the spreadsheet person from everyone else. | Coming soon |
| 29 | PostgreSQL Pro Tips | Shortcuts, settings, and techniques that save time every day The small habits separating slow from fast. | Coming soon |
| 30 | **Capstone:** FinTech Lending Analytics | Build a full analytics platform - schema, pipelines, segmentation, tuning Capstone: FinTech Lending Analytics - the thing most people get wrong. | Coming soon |

---

## Quick Start

**All you need is a computer and an internet connection.** Seriously, that's it.

**Step 1** - Install PostgreSQL & pgAdmin ([watch the setup guide](https://youtu.be/g8GwhsVPaOg) - takes about 10 minutes)

**Step 2** - Clone this repo
```bash
git clone https://github.com/sdw-online/30-Day-SQL-Challenge.git
```

**Step 3** - Create your database
```sql
CREATE DATABASE sql_challenge;
```

**Step 4** - Open [`day-01/`](day-01/) and you're off

---

## What People Are Saying

Real comments from people doing the challenge right now.

---

## Other Installation Guides

Need to set up other tools? These walk you through each one step by step:

| Tool | Guide |
|------|:-----:|
| PostgreSQL & pgAdmin | [Watch](https://youtu.be/g8GwhsVPaOg) |
| MySQL & Workbench | [Watch](https://youtu.be/u8gGLYOhJuA) |
| SQL Server & SSMS | [Watch](https://youtu.be/Th0hB2h7F14) |
| VS Code | [Watch](https://youtu.be/ny-uBrcGP_U) |
| Git | [Watch](https://youtu.be/26moiTYEw6I) |
| Docker | [Watch](https://youtu.be/6KWp6pNAVLU) |

---

## Browse all videos

Click any thumbnail to jump straight to that day's video on YouTube. Sorted by day number.

<table>
<tr>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=mFIMPhiO-N0"><img src="https://img.youtube.com/vi/mFIMPhiO-N0/hqdefault.jpg" width="220"/></a> 
 Day 01 - Introduction to SQL &amp; Databases
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=-0uVBtXCZ_s"><img src="https://img.youtube.com/vi/-0uVBtXCZ_s/hqdefault.jpg" width="220"/></a> 
 Day 02 - SELECT &amp; WHERE
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=s86nI9dPZqY"><img src="https://img.youtube.com/vi/s86nI9dPZqY/hqdefault.jpg" width="220"/></a> 
 Day 03 - ORDER BY &amp; LIMIT
</td>
</tr>
<tr>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=7IWrvTIrIkg"><img src="https://img.youtube.com/vi/7IWrvTIrIkg/hqdefault.jpg" width="220"/></a> 
 Day 04 - Aggregate Functions &amp; GROUP BY
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=NJ4ujmOZt60"><img src="https://img.youtube.com/vi/NJ4ujmOZt60/hqdefault.jpg" width="220"/></a> 
 Day 05 - INSERT, UPDATE &amp; DELETE
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=1AdFU8Vdq-0"><img src="https://img.youtube.com/vi/1AdFU8Vdq-0/hqdefault.jpg" width="220"/></a> 
 Day 06 - Primary &amp; Foreign Keys
</td>
</tr>
<tr>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=fiBYAziNtGI"><img src="https://img.youtube.com/vi/fiBYAziNtGI/hqdefault.jpg" width="220"/></a> 
 Day 07 - Project: Freight &amp; Logistics Report
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=0nH464EoZ9w"><img src="https://img.youtube.com/vi/0nH464EoZ9w/hqdefault.jpg" width="220"/></a> 
 Day 08 - NULL Handling
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=h6J7AajBD6w"><img src="https://img.youtube.com/vi/h6J7AajBD6w/hqdefault.jpg" width="220"/></a> 
 Day 09 - String &amp; Numeric Functions
</td>
</tr>
<tr>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=Iturx2kgs1A"><img src="https://img.youtube.com/vi/Iturx2kgs1A/hqdefault.jpg" width="220"/></a> 
 Day 10 - Date Functions &amp; CAST
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=eZ5iTTsKGkI"><img src="https://img.youtube.com/vi/eZ5iTTsKGkI/hqdefault.jpg" width="220"/></a> 
 Day 11 - CASE WHEN
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=SOt5jUrzKOU"><img src="https://img.youtube.com/vi/SOt5jUrzKOU/hqdefault.jpg" width="220"/></a> 
 Day 12 - Subqueries &amp; Temp Tables
</td>
</tr>
<tr>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=IijQJAfqcJc"><img src="https://img.youtube.com/vi/IijQJAfqcJc/hqdefault.jpg" width="220"/></a> 
 Day 13 - CTEs (Part 1)
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=afIJ4VsQYSo"><img src="https://img.youtube.com/vi/afIJ4VsQYSo/hqdefault.jpg" width="220"/></a> 
 Day 14 - Project: Fleet Intelligence Pipeline
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=wtBxs_iDLo4"><img src="https://img.youtube.com/vi/wtBxs_iDLo4/hqdefault.jpg" width="220"/></a> 
 Day 15 - JOINs Part 1: INNER, LEFT, RIGHT, FULL OUTER
</td>
</tr>
<tr>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=ZYwPGw4ghkI"><img src="https://img.youtube.com/vi/ZYwPGw4ghkI/hqdefault.jpg" width="220"/></a> 
 Day 16 - JOINs Part 2: CROSS &amp; Self Joins
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=wlohArgOSd4"><img src="https://img.youtube.com/vi/wlohArgOSd4/hqdefault.jpg" width="220"/></a> 
 Day 17 - UNION &amp; UNION ALL
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=dhdWwX8DAEg"><img src="https://img.youtube.com/vi/dhdWwX8DAEg/hqdefault.jpg" width="220"/></a> 
 Day 18 - Normalisation and Denormalisation
</td>
</tr>
<tr>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=DI1swwiKxUc"><img src="https://img.youtube.com/vi/DI1swwiKxUc/hqdefault.jpg" width="220"/></a> 
 Day 19 - Recursive CTEs
</td>
<td align="center" width="33%">
 <a href="https://www.youtube.com/watch?v=bZjfSWOIsBI"><img src="https://img.youtube.com/vi/bZjfSWOIsBI/hqdefault.jpg" width="220"/></a> 
 Day 20 - Data Modelling and Star Schema
</td>
</tr>
</table>

---

## Join the community

<a href="https://data100x.carrd.co/">Join the free data community →</a> 
 Weekly drops, member projects, no spam, 100% free.

Want the deeper material? <a href="https://davidwilliams2.gumroad.com/l/cxypl">SQL Made Easy</a> on Gumroad.

---

You just mass-gained a skill most people spend months fumbling through. Stephen drops new challenges, projects, and deep dives regularly - [subscribe on YouTube](https://www.youtube.com/@sdw-online?sub_confirmation=1) so you don't miss the next one.

That's how people go from "learning SQL" to being the SQL person on the team.

---

## About

I'm Stephen - a Senior Data Engineer who's worked across consulting, startups, and enterprise (BDO, TCS, easyJet, C. Hoare & Co., Veolia UK/US).

I've spent years doing this work professionally, and I created this challenge to pass on what I've learnt - the practical stuff, not the theory. The kind of SQL you actually write on the job. Thousands of people have gone through the Excel and SQL challenges already, and watching people message me saying they got their first data role because of these videos is genuinely why I keep doing it.

**YouTube:** [Stephen | Data](https://www.youtube.com/@sdw-online?sub_confirmation=1)

---

## License

For educational purposes. Fork it, clone it, learn from it. If you share it, a link back is appreciated.

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