GROUP BY single column in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the GROUP BY clause do in SQL?

It groups rows that have the same value in a specified column into summary rows, like grouping all sales by each product.

beginner

Write a simple SQL query to count how many orders each customer has made using GROUP BY on a single column.

SELECT customer_id, COUNT(*) AS order_count FROM orders GROUP BY customer_id;

beginner

Why do we need to use GROUP BY when using aggregate functions like COUNT() or SUM()?

Because aggregate functions summarize data, GROUP BY tells SQL how to group rows before summarizing, so you get results per group instead of one total.

intermediate

Can you use GROUP BY on multiple columns? What happens if you only use one column?

Yes, you can group by multiple columns to get more detailed groups. Using one column groups rows only by that column's values, combining all other differences.

intermediate

What will happen if you select columns in a query with GROUP BY but don't include them in the GROUP BY clause or an aggregate function?

SQL will give an error because it doesn't know how to group or summarize those columns. Every selected column must be grouped or aggregated.

What does the SQL clause GROUP BY do when used with a single column?

A Sorts the rows by that column

B Deletes duplicate rows from the table

C Groups rows with the same value in that column into summary rows

D Filters rows based on a condition

GROUP BY groups rows sharing the same value in the specified column, allowing aggregate functions to summarize each group.

Which aggregate function counts the number of rows in each group?

A SUM()

B COUNT()

C AVG()

D MAX()

COUNT() counts the number of rows in each group.

What will this query return?
SELECT category, COUNT(*) FROM products GROUP BY category;

A Total count of all products

B Error because COUNT(*) is used

C List of categories without counts

D Count of products per category

The query groups products by category and counts how many products are in each category.

If you select a column not in the GROUP BY clause or an aggregate function, what happens?

A SQL returns an error

B SQL ignores that column

C SQL returns the first value it finds

D SQL groups by that column automatically

SQL requires all selected columns to be either grouped or aggregated; otherwise, it throws an error.

Which of these is a valid use of GROUP BY on a single column?

A SELECT name, SUM(price) FROM sales GROUP BY name;

B SELECT name, SUM(price) FROM sales;

C SELECT SUM(price) FROM sales GROUP BY name;

D SELECT name FROM sales GROUP BY price;

Option A groups sales by name and sums prices per name, which is valid.

Explain in your own words how the GROUP BY clause works when grouping by a single column.

Think about how you might group items in real life, like sorting fruits by type.

Describe a simple example where you would use GROUP BY on one column and an aggregate function in a query.

Imagine counting how many orders each customer made.

Practice

1. What does the GROUP BY clause do in an SQL query?

easy

A. It deletes duplicate rows from the table.

B. It sorts the rows in ascending order.

C. It groups rows that have the same values in a specified column.

D. It filters rows based on a condition.

Solution

1. Step 1: Understand the purpose of GROUP BY

   The GROUP BY clause collects rows with the same value in the specified column into groups.

2. Step 2: Differentiate from other clauses

   Unlike ORDER BY which sorts, or WHERE which filters, GROUP BY organizes data for aggregation.

3. Final Answer:

   It groups rows that have the same values in a specified column. -> Option C

4. Quick Check:

   GROUP BY = grouping rows by column [OK]

Hint: GROUP BY collects rows sharing column values [OK]

Common Mistakes:

• Confusing GROUP BY with ORDER BY
• Thinking GROUP BY filters rows
• Assuming GROUP BY deletes duplicates

2. Which of the following is the correct syntax to group data by the column department?

easy

A. SELECT department, COUNT(*) FROM employees GROUP BY department;

B. SELECT department, COUNT(*) FROM employees ORDER BY department;

C. SELECT department, COUNT(*) FROM employees WHERE department;

D. SELECT department, COUNT(*) FROM employees HAVING department;

Solution

1. Step 1: Identify correct GROUP BY usage

   The GROUP BY clause must follow FROM and group by the column named.

2. Step 2: Check each option's clause

   SELECT department, COUNT(*) FROM employees GROUP BY department; uses GROUP BY correctly; others use ORDER BY, WHERE, HAVING incorrectly here.

3. Final Answer:

   SELECT department, COUNT(*) FROM employees GROUP BY department; -> Option A

4. Quick Check:

   GROUP BY syntax = SELECT ... GROUP BY column [OK]

Hint: GROUP BY follows FROM and groups by column [OK]

Common Mistakes:

• Using ORDER BY instead of GROUP BY
• Using WHERE to group data
• Using HAVING without aggregation

3. Given the table sales with columns region and amount, what is the output of this query?

SELECT region, SUM(amount) FROM sales GROUP BY region;

medium

A. A list of all sales amounts without grouping.

B. A list of regions with the total sales amount for each region.

C. An error because SUM() cannot be used with GROUP BY.

D. A list of regions sorted by amount.

Solution

1. Step 1: Understand GROUP BY with SUM()

   The query groups rows by region and sums the amount for each group.

2. Step 2: Predict output format

   The output shows each region once with the total amount of sales in that region.

3. Final Answer:

   A list of regions with the total sales amount for each region. -> Option B

4. Quick Check:

   GROUP BY + SUM() = grouped sums [OK]

Hint: GROUP BY + SUM() gives totals per group [OK]

Common Mistakes:

• Thinking SUM() can't be used with GROUP BY
• Expecting ungrouped list
• Confusing sorting with grouping

4. Identify the error in this SQL query:

SELECT department, COUNT(*) FROM employees;

medium

A. The query is correct and will run without errors.

B. COUNT(*) cannot be used without WHERE clause.

C. department cannot be selected without aggregation.

D. Missing GROUP BY clause for the department column.

Solution

1. Step 1: Check SELECT with aggregation

   COUNT(*) is an aggregate but department is not aggregated or grouped.

2. Step 2: Identify missing GROUP BY

   To select department with COUNT(*), GROUP BY department is required.

3. Final Answer:

   Missing GROUP BY clause for the department column. -> Option D

4. Quick Check:

   Non-aggregated columns need GROUP BY [OK]

Hint: Non-aggregated columns need GROUP BY [OK]

Common Mistakes:

• Ignoring missing GROUP BY
• Thinking COUNT(*) needs WHERE
• Assuming query runs without error

5. You have a table orders with columns customer_id, order_date, and total. You want to find the average order total per customer but only for customers who have placed more than 3 orders. Which query correctly achieves this?

hard

A. SELECT customer_id, AVG(total) FROM orders GROUP BY customer_id HAVING COUNT(*) > 3;

B. SELECT customer_id, AVG(total) FROM orders WHERE COUNT(*) > 3 GROUP BY customer_id;

C. SELECT customer_id, AVG(total) FROM orders GROUP BY customer_id WHERE COUNT(*) > 3;

D. SELECT customer_id, AVG(total) FROM orders HAVING COUNT(*) > 3 GROUP BY customer_id;

Solution

1. Step 1: Use GROUP BY to group orders by customer_id

   This groups all orders per customer to calculate aggregates.

2. Step 2: Use HAVING to filter groups with more than 3 orders

   HAVING filters groups after aggregation; WHERE cannot filter aggregates.

3. Final Answer:

   SELECT customer_id, AVG(total) FROM orders GROUP BY customer_id HAVING COUNT(*) > 3; -> Option A

4. Quick Check:

   HAVING filters groups, WHERE filters rows [OK]

Hint: Use HAVING to filter groups after GROUP BY [OK]

Common Mistakes:

• Using WHERE to filter aggregated counts
• Placing HAVING before GROUP BY
• Not filtering groups at all