MySQL GROUP BY

 MySQL GROUP BY

Summary: in this tutorial, you will learn how to use MySQL GROUP BY to 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.

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 information about each group.

The GROUP BY a 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;

The GROUP BY the clause must appear after the FROM  WHERE clauses. Following the GROUP BY keyword is a list of comma-separated columns or expressions that you want to use as criteria to group rows.

MySQL evaluates the GROUP BY clause after the FROM, WHERE and SELECT clauses and before the HAVING , ORDER BY and LIMIT clauses:

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 the 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;

As you can see, 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;

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 a 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;

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;

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;

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);

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;

The GROUP BY clause: MySQL vs. standard SQL

Standard SQL does not allow you to use an alias in the GROUP BY clause, however, MySQL supports this.

For example, the following query extracts the year from the order date. It first uses year as an alias of the expression YEAR(orderDate) and then uses the year alias in the GROUP BY clause. This query is not valid in standard SQL.

SELECT 
    YEAR(orderDate) AS year, 
    COUNT(orderNumber)
FROM
    orders
GROUP BY 
    year;

MySQL also allows you to sort the groups in ascending or descending orders while the standard SQL does not. The default 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;

Notice that we used DESC in the GROUP BY clause to sort the status in descending order. We could also specify explicitly ASC in the GROUP BY clause to sort the groups by status in ascending order.

In this tutorial, we have shown you how to use the MySQL GROUP BY clause to group rows into subgroups based on values of columns or expressions.