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?<br>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.