GROUP BY single and multiple columns in PostgreSQL - 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 finding the total sales per product.

beginner

How do you use GROUP BY with a single column?

You list one column after GROUP BY to group rows by that column's values. For example, GROUP BY department groups all rows by each department.

intermediate

How does GROUP BY work with multiple columns?

You list multiple columns separated by commas after GROUP BY. This groups rows by unique combinations of those columns. For example, GROUP BY city, department groups rows by each city and department pair.

beginner

Why do you often use aggregate functions with GROUP BY?

Because GROUP BY creates groups, aggregate functions like SUM(), COUNT(), or AVG() calculate summary values for each group.

intermediate

What happens if you select columns not in the GROUP BY clause or aggregate functions?

PostgreSQL will give an error because every selected column must be either grouped or aggregated to avoid confusion about which row's value to show.

What does GROUP BY department do?

A Groups rows by each unique department

B Sorts rows by department

C Deletes duplicate departments

D Filters rows with department

GROUP BY department groups rows that share the same department value into one group.

Which query groups by both city and department?

A GROUP BY city; GROUP BY department

B GROUP BY city OR department

C GROUP BY city, department

D GROUP BY city AND department

Use commas to list multiple columns in GROUP BY, like GROUP BY city, department.

Which aggregate function counts rows in each group?

A COUNT()

B MAX()

C AVG()

D SUM()

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

What error occurs if you select a column not in GROUP BY or aggregated?

A Syntax error

B Column must appear in the GROUP BY clause or be used in an aggregate function

C Missing FROM clause

D No error

PostgreSQL requires all selected columns to be grouped or aggregated to avoid ambiguity.

What is the result of GROUP BY?

A A single row

B Rows deleted

C Rows sorted alphabetically

D Rows grouped into summary rows

GROUP BY groups rows into summary rows based on column values.

Explain how to use GROUP BY with one and multiple columns in PostgreSQL.

Think about grouping by one column versus grouping by pairs or more.

Why must columns in SELECT be either in GROUP BY or aggregated?

Consider what happens if you select a column not grouped or aggregated.