# Exercise: Aggregating Data

Twelve questions about Northwind, answered with COUNT, SUM, AVG, MIN, MAX, GROUP BY and HAVING, each with the result to expect.

Journeyman · T-SQL · SQL · Updated October 30, 2026

Northwind's managers have questions: how much stock is there, who are the best customers, which orders were the biggest? Each one is a single query, and each needs an aggregate.

## Get the exercise code

Open this lesson's GitHub repository, [github.com/ask-codey/exercise-aggregating-data](https://github.com/ask-codey/exercise-aggregating-data), also linked from the **Exercise** box beside the lesson. Click **Code**, then **Download ZIP**, and unzip it into your `bootcamp` folder. Or clone it with Git.

There's one `.sql` file per question, numbered in order. Each holds the question as a comment, and an **Expect** line that says what the right answer looks like. Nothing here changes any data.

## What to do

1. Open SQL Server Management Studio, and connect to `localhost`.
2. Choose **File > Open > File...** and open `01_Count_Products.sql`.
3. Choose **Northwind** in the database drop-down.
4. Write your query under the question, select it, and press **F5**.
5. Compare your result with the **Expect** line, then go on to the next file.

| Questions | Practice |
|---|---|
| 1–6 | One aggregate for the whole table: `COUNT`, `SUM`, `AVG`, `MIN`, `MAX`, and `COUNT(DISTINCT ...)` |
| 7–8 | One row per group with `GROUP BY` |
| 9–11 | Filtering groups with `HAVING`, and rows with `WHERE`, in the same query |
| 12 | Bonus: an aggregate of a calculation |

## Hints

- **One row, or one per something?** One row for the whole table is an aggregate on its own; one row per year, per category or per customer is `GROUP BY`.
- **Which filter?** Rows before counting: `WHERE`. Groups by their count or total: `HAVING`.
- **`NULL` counts differently.** `COUNT(*)` counts rows; `COUNT(column)` skips `NULL`.
- **Money can show extra zeros.** Some tools, such as sqlcmd, show money with four decimal places: 17250.0000 is the same answer as the 17250.00 in an **Expect** line.

## When you're done

Every result matches its **Expect** line. Look back at questions 9 and 11: they're the queries that turn a table of 830 orders into a short list worth acting on.

## Submit your exercise

The exercise is how you finish this lesson. When your work is done, put it on GitHub in a repo of your own, then submit that repo's address here.

1. On GitHub, click **+**, then **New repository**. Name it (for example `exercise-aggregating-data`), choose **Public**, and click **Create repository**. Leave out the README: your folder already has one.
2. In a terminal in your exercise folder (Git Bash on Windows), send your work to your new repo. If you downloaded the ZIP:

   ```bash
   git init -b main
   git add .
   git commit -m "My solution"
   git remote add origin https://github.com/your-name/exercise-aggregating-data.git
   git push -u origin main
   ```

   If you cloned the starter instead, point your copy at your own repo, then commit and push:

   ```bash
   git remote set-url origin https://github.com/your-name/exercise-aggregating-data.git
   git add .
   git commit -m "My solution"
   git push -u origin main
   ```

3. Paste your repo's address, such as `https://github.com/your-name/exercise-aggregating-data`, into **Submit your exercise** on this page. The site checks that the repo is public and holds your commits, and then the lesson is complete.

Changed something after you submitted? Push it, and submit again: your latest submission is the one that counts.

## So what

Counting, totaling and ranking groups is most of what reports are made of. You can now answer a manager's "how many, how much, and which ones" in a single query.

## Key terms

- [Aggregate Function](https://www.ask-codey.com/glossary/aggregate-function)
- [COUNT](https://www.ask-codey.com/glossary/count)
- [GROUP BY](https://www.ask-codey.com/glossary/group-by)
- [HAVING](https://www.ask-codey.com/glossary/having)

## Related

- [Common Aggregate Functions](https://www.ask-codey.com/lessons/aggregate-functions-in-t-sql-common-aggregate-functions)
- [Grouping Aggregations](https://www.ask-codey.com/lessons/aggregate-functions-in-t-sql-grouping-aggregations)
- [HAVING](https://www.ask-codey.com/lessons/aggregate-functions-in-t-sql-having)

Source: https://www.ask-codey.com/lessons/exercise-aggregating-data