JOIN with aggregate functions in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using JOIN with aggregate functions in SQL?

JOIN combines rows from two or more tables based on a related column, and aggregate functions calculate a summary value (like sum, count) over groups of rows. Together, they help analyze related data across tables.

beginner

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

The COUNT() function counts the number of rows in a group, including or excluding NULLs depending on usage.

intermediate

How do you write a JOIN query that sums a column from a related table?

Use JOIN to connect tables, then use SUM() on the related column, and GROUP BY to group results by the main table's key.

beginner

Why do you need GROUP BY when using aggregate functions with JOIN?

GROUP BY groups rows by one or more columns so aggregate functions can calculate summaries per group instead of over the entire result set.

intermediate

What happens if you forget GROUP BY when using aggregate functions with JOIN?

The aggregate function will calculate a single summary for all joined rows, losing detail per group, often leading to incorrect or unexpected results.

What does the COUNT() function do in a JOIN query?

A Counts the number of rows in each group

B Joins two tables together

C Calculates the average of a column

D Deletes duplicate rows

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

Which clause is necessary to group rows when using SUM() with JOIN?

A WHERE

B ORDER BY

C HAVING

D GROUP BY

GROUP BY groups rows so SUM() can calculate totals per group.

If you want to find total sales per customer, which SQL elements do you need?

A JOIN, SUM(), GROUP BY

B JOIN, COUNT(), ORDER BY

C WHERE, AVG(), GROUP BY

D JOIN, MAX(), HAVING

JOIN connects customers and sales tables, SUM() totals sales, GROUP BY groups by customer.

What does the HAVING clause do in a JOIN with aggregate functions?

A Filters rows before grouping

B Filters groups after aggregation

C Sorts the result set

D Joins tables

HAVING filters groups after aggregate functions are applied.

Which JOIN type includes all rows from the left table even if there is no match in the right table?

A INNER JOIN

B RIGHT JOIN

C LEFT JOIN

D FULL JOIN

LEFT JOIN includes all rows from the left table and matched rows from the right.

Explain how to use JOIN with aggregate functions to calculate total values per group.

Think about combining tables and then summarizing data by groups.

Describe why GROUP BY is important when using aggregate functions with JOIN.

Consider what happens if you aggregate without grouping.