Combining multiple aggregates in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the term aggregate function mean in SQL?

An aggregate function performs a calculation on a set of values and returns a single value, like SUM, COUNT, AVG, MIN, or MAX.

beginner

How can you combine multiple aggregate functions in one SQL query?

You can list multiple aggregate functions separated by commas in the SELECT clause, for example: SELECT COUNT(*), AVG(price) FROM products;

intermediate

Why do we use GROUP BY with aggregate functions?

GROUP BY groups rows that have the same values in specified columns so aggregate functions can calculate results for each group separately.

intermediate

What will this query return? <br>SELECT department, COUNT(*), AVG(salary) FROM employees GROUP BY department;

It returns each department's name, the number of employees in that department, and the average salary of employees in that department.

intermediate

Can you combine aggregate functions without GROUP BY? What happens?

Yes, you can combine aggregates without GROUP BY. The functions will calculate over the entire table and return one row with combined results.

Which SQL clause is used to group rows for aggregate calculations?

A GROUP BY

B ORDER BY

C WHERE

D HAVING

GROUP BY groups rows so aggregate functions can calculate results per group.

What does this query return? <br>SELECT COUNT(*), SUM(quantity) FROM sales;

A Average quantity and total sales

B Number of unique products and total quantity

C Number of sales rows and total quantity sold

D List of sales rows

COUNT(*) counts rows, SUM(quantity) adds all quantity values.

Can you use multiple aggregate functions in one SELECT statement?

A Only if you use subqueries

B No, only one aggregate per query

C Only with JOINs

D Yes, separated by commas

You can list multiple aggregates separated by commas in SELECT.

What happens if you use aggregate functions without GROUP BY?

A It groups by all columns automatically

B Aggregates calculate over the whole table and return one row

C Aggregates calculate per each row

D Query returns an error

Without GROUP BY, aggregates summarize the entire table.

Which of these is NOT an aggregate function?

A SELECT

B MAX

C COUNT

D SUM

SELECT is a SQL command, not an aggregate function.

Explain how to combine multiple aggregate functions in a single SQL query and when to use GROUP BY.

Think about how to get counts and averages per category.

Describe what happens when you use aggregate functions without a GROUP BY clause.

Consider the whole table as one group.