MySQL GROUP BY
Summary: in this tutorial, you will learn how to use MySQL GROUP BYto group rows into subgroups based on values of columns or expressions.
Introduction to MySQL GROUP BY clause
The GROUP BY clause groups a set of rows into a set of summary rows by values of columns or expressions. The GROUP BY clause returns one row for each group. In other words, it reduces the number of rows in the result set.
The GROUP BY clause is an optional clause of the SELECT statement. The following illustrates the GROUP BY clause syntax:
SELECT
c1, c2,..., cn, aggregate_function(ci)
FROM
table
WHERE
where_conditions
GROUP BY c1 , c2,...,cn;Code language: SQL (Structured Query Language) (sql)
In this syntax, you place the GROUP BY clause after the FROM and WHERE clauses. After the GROUP BY keywords, you place is a list of comma-separated columns or expressions to group rows.
MySQL evaluates the GROUP BY clause after the FROM and WHERE clauses and before the HAVING, SELECT, DISTINCT, ORDER BY and LIMIT clauses:
In practice, you often use the GROUP BY clause with aggregate functions such as SUM, AVG, MAX, MIN, and COUNT. The aggregate function that appears in the SELECT clause provides the information of each group.
MySQL GROUP BY examples
Let’s take some examples of using the GROUP BY clause.
A) Simple MySQL GROUP BY example
Let’s take a look at the orders table in the sample database.
Suppose you want to group values of the order’s status into subgroups, you use the GROUP BY clause with the status column as the following query:
SELECT
status
FROM
orders
GROUP BY status;Code language: SQL (Structured Query Language) (sql)
As you can see clearly from the output, the GROUP BY clause returns unique occurrences of status values. It works like the DISTINCT operator as shown in the following query:
SELECT DISTINCT
status
FROM
orders;Code language: SQL (Structured Query Language) (sql)
B) Using MySQL GROUP BY with aggregate functions
The aggregate functions allow you to perform the calculation of a set of rows and return a single value. The GROUP BY clause is often used with an aggregate function to perform calculations and return a single value for each subgroup.
For example, if you want to know the number of orders in each status, you can use the COUNT function with the GROUP BY clause as follows:
SELECT
status, COUNT(*)
FROM
orders
GROUP BY status;Code language: SQL (Structured Query Language) (sql)
See the following orders and orderdetails table.
To get the total amount of all orders by status, you join the orders table with the orderdetails table and use the SUM function to calculate the total amount. See the following query:
SELECT
status,
SUM(quantityOrdered * priceEach) AS amount
FROM
orders
INNER JOIN orderdetails
USING (orderNumber)
GROUP BY
status;Code language: SQL (Structured Query Language) (sql)
Similarly, the following query returns the order numbers and the total amount of each order.
SELECT
orderNumber,
SUM(quantityOrdered * priceEach) AS total
FROM
orderdetails
GROUP BY
orderNumber;Code language: SQL (Structured Query Language) (sql)
C) MySQL GROUP BY with expression example
In addition to columns, you can group rows by expressions. The following query gets the total sales for each year.
SELECT
YEAR(orderDate) AS year,
SUM(quantityOrdered * priceEach) AS total
FROM
orders
INNER JOIN orderdetails
USING (orderNumber)
WHERE
status = 'Shipped'
GROUP BY
YEAR(orderDate);Code language: SQL (Structured Query Language) (sql)
In this example, we used the YEAR function to extract year data from order date ( orderDate). We included only orders with shipped status in the total sales. Note that the expression which appears in the SELECT clause must be the same as the one in the GROUP BY clause.
D) Using MySQL GROUP BY with HAVING clause example
To filter the groups returned by GROUP BY clause, you use a HAVING clause. The following query uses the HAVING clause to select the total sales of the years after 2003.
SELECT
YEAR(orderDate) AS year,
SUM(quantityOrdered * priceEach) AS total
FROM
orders
INNER JOIN orderdetails
USING (orderNumber)
WHERE
status = 'Shipped'
GROUP BY
year
HAVING
year > 2003;Code language: SQL (Structured Query Language) (sql)
The GROUP BY clause: MySQL vs. SQL standard
The SQL standard does not allow you to use an alias in the GROUP BY clause whereas MySQL supports this.
For example, the following query extracts the year from the order date. It first uses the year as an alias of the expression YEAR(orderDate) and then uses the year alias in the GROUP BY clause.
The following query is not valid in SQL standard:
SELECT
YEAR(orderDate) AS year,
COUNT(orderNumber)
FROM
orders
GROUP BY
year;Code language: SQL (Structured Query Language) (sql)
Also, MySQL allows you to sort the groups in ascending or descending orders. The default sorting order is ascending. For example, if you want to get the number of orders by status and sort the status in descending order, you can use the GROUP BY clause with DESC as the following query:
SELECT
status,
COUNT(*)
FROM
orders
GROUP BY
status DESC;Code language: SQL (Structured Query Language) (sql)
Notice the DESC in the GROUP BY clause sorts the status in descending order. And you can also use the ASC explicitly in the GROUP BY clause to sort the groups by status in ascending order.
The GROUP BY clause vs. DISTINCT clause
If you use the GROUP BY clause in the SELECT statement without using aggregate functions, the GROUP BY clause behaves like the DISTINCT clause.
The following statement uses the GROUP BY clause to select the unique states of customers from the customers table.
SELECT
state
FROM
customers
GROUP BY state;Code language: SQL (Structured Query Language) (sql)
You can achieve a similar result by using the DISTINCT clause:
SELECT DISTINCT
state
FROM
customers;Code language: SQL (Structured Query Language) (sql)
Generally speaking, the DISTINCT clause is a special case of the GROUP BY clause. The difference between DISTINCT clause and GROUP BY clause is that the GROUP BY clause sorts the result set, whereas the DISTINCT clause does not.
Notice that MySQL 8.0 removed the implicit sorting for the GROUP BY clause. Therefore, if you use MySQL 8.0+, you will find that the result set of the above query with the GROUP BY clause is not sorted.
If you add the ORDER BY clause to the statement that uses the DISTINCT clause, the result set is sorted, and it is the same as the one returned by the statement that uses GROUP BY clause.
SELECT DISTINCT
state
FROM
customers
ORDER BY
state;Code language: SQL (Structured Query Language) (sql)
Summary
- Use the
GROUP BYclause to group rows into subgroups.