# GROUP BY in SQL Explained

*The SQL GROUP BY statement is easy to use, but it can be hard to master. Learn what you can do with GROUP BY, how it works, and how to use it with aggregate functions.*

GROUP BY is one of the basic SQL tools. However, it might be difficult for beginners to understand how `GROUP BY`

works and how it is used to compute statistics with different aggregate functions. In this article, I’ll use several examples to show exactly how GROUP BY organizes rows and how aggregate functions can be used to calculate statistics on one or more columns.

So, let’s get started.

## How Does SQL GROUP BY Work?

`GROUP BY`

is an indispensable tool for any data analyst working with SQL. If you want to organize your data in groups and calculate some kind of aggregate statistics for these groups, the `GROUP BY`

clause is what you need.

So, what does `GROUP BY`

do? Basically, it **groups rows with the same value into one group or bucket**. Let’s see how it works.

Suppose we’re running a bookstore and want to know how many books of different genres we have in stock. Our database includes a table that lists the books’ titles, genres, and stock quantity.

The visualization below shows how the `GROUP BY`

clause creates groups from table data. We want to know the total quantity of books for each genre; thus, `GROUP BY`

**groups the books of the same genre and sums up the corresponding quantities**. This creates a result table that lists genres and their total quantity of books in our stock.

For a more detailed introduction to the `GROUP BY`

topic, see this video tutorial from our “We Learn SQL” series.

## GROUP BY in Action

Now it’s time for more specific examples of SQL queries with a `GROUP BY`

clause. We’ll use the ** books** table, which stores the ID, title, author, genre, language, price, and quantity of each novel we stock.

id | title | author | genre | lang | price | qty |
---|---|---|---|---|---|---|

1 | Les Trois Mousquetaires | Alexandre Dumas | adventure | fr | 11.90 | 4 |

2 | A Game of Thrones | George R.R. Martin | fantasy | en | 8.49 | 5 |

3 | Pride and Prejudice | Jane Austen | romance | en | 9.99 | 2 |

4 | Vampire Academy | Richelle Mead | fantasy | en | 7.99 | 3 |

5 | Ivanhoe | Walter Scott | adventure | en | 9.99 | 3 |

6 | Armance | Stendhal | romance | fr | 5.88 | 1 |

To calculate the total number of books of each genre, we’ll use the following query:

SELECT genre, SUM(qty) AS total FROM books GROUP BY genre;

Here, in the `GROUP BY`

clause, we select our rows to be grouped by the column genre. Then the `SUM(qty)`

function in the `SELECT`

statement sums the qty values within each group (i.e. each book genre), with the result displayed in the corresponding `total`

field:

genre | total |
---|---|

adventure | 7 |

fantasy | 8 |

romance | 3 |

## Using Aggregate Functions with GROUP BY

`GROUP BY`

puts rows with the same value into one bucket. We usually want to compute some statistics for this group of rows, like the average value or the total quantity. To this end, SQL provides aggregate functions that combine values from a certain column into one value for the respective group.

So far, we’ve only used `SUM()`

as our aggregate function for grouping the book titles in stock. However, this is not the only aggregate function you can use with `GROUP BY`

. SQL also offers:

`COUNT()`

to calculate the number of rows in each group.`AVG()`

to find the average value for each group.`MIN()`

to return the minimum value in each group.`MAX()`

to return the maximum value in each group.

Let’s see how the `AVG()`

function works with `GROUP BY`

. This time, we want to calculate the average price for books in each genre. We’ll start by visualizing the output we want to get.

We’re again grouping our books by genre, but this time we want to calculate the average book price in each genre. The SQL query for this looks as follows:

SELECT genre, AVG(price) AS avg_price FROM books GROUP BY genre;

This query creates a table with two columns (genre and `avg_price`

), where the average price is calculated by averaging the price values for the books of each genre:

genre | avg_price |
---|---|

adventure | 10.945 |

fantasy | 8.24 |

romance | 7.935 |

Actually, we are **not limited to using only one aggregate function** with a `GROUP BY`

clause. So, let’s add the information about the minimum and maximum price of the books in each genre:

SELECT genre, MIN(price) AS min_price, AVG(price) AS avg_price, MAX(price) AS max_price FROM books GROUP BY genre;

The result set now includes four columns: `genre`

, `min_price`

, `avg_price`

, and `max_price`

.

genre | min_price | avg_price | max_price |
---|---|---|---|

adventure | 9.99 | 10.945 | 11.90 |

fantasy | 7.99 | 8.24 | 8.49 |

romance | 5.88 | 7.935 | 9.99 |

Note that when using `GROUP BY`

**the SELECT statement may only include fields that are either used in the aggregate function or listed in the GROUP BY clause**. For example, in our case, we cannot add

`title`

or author to our result set. To do so makes no sense, as each row in our output table includes information on several books with different titles and authors.To learn about other problems you may encounter when using `GROUP BY`

in SQL, check out this article that discusses common GROUP BY errors and how to avoid them.

## GROUP BY Two Columns

In SQL, you can also group your data using several columns. For example, let’s say we want to group our books not only by genre but also by language. And we want to calculate the number of book titles in each category.

**Hint:** The `COUNT()`

aggregate function will assist us in calculating the number of book titles (i.e. rows).

To compute the number of book titles by genre and language, we’ll use the following query:

SELECT genre, lang, count(title) AS titles FROM books GROUP BY genre, lang;

We’ll get an output table with three columns: `genre`

, language (`lang`

), and the number of `titles`

in that category:

genre | lang | titles |
---|---|---|

adventure | en | 1 |

adventure | fr | 1 |

fantasy | en | 2 |

romance | en | 1 |

romance | fr | 1 |

So, in our example dataset, we have:

- One
**English-languag**e book title in the**adventure**genre (*Ivanhoe*). - One
**French-language**book title in the**adventure**genre (*Les Trois Mousquetaires*). - Two
**English-language**book titles in the**fantasy**genre (*A Game of Thrones and Vampire Academy*). - One
**English-language**book title in the**romance**genre (*Pride and Prejudice*). - One
**French-language**book title in the**romance**genre (*Armance*).

As you see, there are no **French-language** fantasy books in our dataset.

For more examples of `GROUP BY`

applications, check out this article showing how GROUP BY can be leveraged in realistic business scenarios.

## Ready to Use GROUP BY in SQL?

Now you have a better understanding of how `GROUP BY`

can assist you with organizing and analyzing your data. However, if you want to handle `GROUP BY`

like a pro, you need lots of practice.

LearnSQL.com offers an SQL Basics course that includes 129 interactive exercises covering aggregation, grouping, JOINs, subqueries, and more. This course is a perfect opportunity to learn how to group rows and compute statistics with aggregate functions.

Happy learning!