GROUP BY with multiple columns in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the GROUP BY clause do in SQL?

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

beginner

How do you use GROUP BY with multiple columns?

You list the columns separated by commas after GROUP BY. This groups rows by unique combinations of those columns.

intermediate

Why would you group by multiple columns instead of just one?

Grouping by multiple columns lets you see detailed summaries for each unique combination, like sales by product and region together.

beginner

Example: What does this query do?<br>

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

It counts how many employees work in each department within each city, grouping by both city and department.

intermediate

Can you use ORDER BY with GROUP BY on multiple columns?

Yes, you can order the grouped results by one or more columns to sort the summary rows as you want.

What happens if you use GROUP BY city, department in a query?

A Rows are grouped by unique city and department pairs.

B Rows are grouped only by city.

C Rows are grouped only by department.

D Rows are not grouped at all.

Using multiple columns in GROUP BY groups rows by each unique combination of those columns.

Which SQL clause is used to summarize data by groups?

A JOIN

B WHERE

C ORDER BY

D GROUP BY

GROUP BY groups rows to allow aggregation functions like COUNT or SUM on each group.

If you want to count employees per city and department, which is correct?

A GROUP BY city

B GROUP BY city, department

C GROUP BY department

D GROUP BY COUNT(*)

Grouping by both city and department counts employees for each unique city-department pair.

Can you use aggregate functions like COUNT() with GROUP BY multiple columns?

A Aggregate functions are not related to GROUP BY.

B No, aggregate functions only work without GROUP BY.

C Yes, aggregate functions work on each group.

D Only COUNT() works, others don't.

Aggregate functions summarize data within each group defined by GROUP BY.

What does this query return?<br>

SELECT product, region, SUM(sales) FROM sales_data GROUP BY product, region;

A Total sales per product and region combination.

B Total sales per product only.

C Total sales per region only.

D All sales data without grouping.

The query sums sales grouped by each product and region pair.

Explain how GROUP BY with multiple columns works and why it is useful.

Think about grouping data like sorting mail by city and street together.

Write a simple SQL query using GROUP BY with two columns and explain what it does.

Use example like counting employees by city and department.