MySQL GROUP_CONCAT Function

 MySQL GROUP_CONCAT Function

summary: in this tutorial, you will learn how to use the MySQL GROUP_CONCAT() function to concatenate strings from a group with various options.

Introduction to MySQL GROUP_CONCAT() function

The MySQL GROUP_CONCAT() the function is an aggregate function that concatenates strings from a group into a single string with various options.

The following shows the syntax of the GROUP_CONCAT() function:

GROUP_CONCAT(
    DISTINCT expression
    ORDER BY expression
    SEPARATOR sep
);

The following example demonstrates how the GROUP_CONCAT() function works.

CREATE TABLE t (
    v CHAR
);

INSERT INTO t(v) VALUES('A'),('B'),('C'),('B');

SELECT 
    GROUP_CONCAT(DISTINCT v
        ORDER BY v ASC
        SEPARATOR ';')
FROM
    t;

The DISTINCT the clause allows you to eliminate duplicate values in the group before concatenating them.

The ORDER BY the clause allows you to sort the values in ascending or descending order before concatenating. By default, it sorts the values in ascending order. If you want to sort the values in descending order, you need to specify explicitly the DESC option.

The SEPARATOR specifies a literal value inserted between values in the group. If you do not specify a separator, the GROUP_CONCAT function uses a comma (,) as the default separator.

The GROUP_CONCAT function ignores NULL values. It returns NULL if there was no matching row found or all arguments are NULL values.

The GROUP_CONCAT the function returns a binary or non-binary string, which depends on the arguments. by default, the maximum length of the return string is 1024. In case you need more than this, you can extend the maximum length by setting the group_concat_max_len system variable at SESSION or GLOBAL level.

MySQL GROUP_CONCAT() function examples

Let’s take a look at the customers table in the sample database.

To get all countries where customers locate as a comma-separated string, you use the GROUP_CONCAT() function as follows:

SELECT 
    GROUP_CONCAT(country)
FROM
    customers;

However, some customers are located in the same country. To remove the duplicate country’s names, you add the DISTINCT a clause like the following query:

SELECT 
    GROUP_CONCAT(DISTINCT country)
FROM
    customers;

It is more readable if the country’s names are in ascending order. To sort the country’s name before concatenating, you use the ORDER BY clause as follows:

SELECT 
    GROUP_CONCAT(DISTINCT country
        ORDER BY country)
FROM
    customers;

To change the default separator of the returned string from a comma (,) to a semi-colon (;), you use the SEPARATOR a clause like the following query:

SELECT 
    GROUP_CONCAT(DISTINCT country
        ORDER BY country
        SEPARATOR ';')
FROM
    customers;

Great! now you know how the GROUP_CONCAT() function works. Let’s put it in a practical example.

Each customer has one or more sales representatives. In other words, each sales employee is in charge of one or more customers. To find out who is in charge of which customers, you use the inner join clause as follows:

SELECT 
    employeeNumber, 
    firstname, 
    lastname, 
    customername
FROM
    employees
        INNER JOIN
    customers ON customers.salesRepEmployeeNumber = employees.employeeNumber
ORDER BY 
	firstname, 
    lastname;

Now, we can group the result set by the employee number and concatenate all employees that are being in charge of the employee by using the GROUP_CONCAT() function as follows:

SELECT 
    employeeNumber,
    firstName,
    lastName,
    GROUP_CONCAT(DISTINCT customername
        ORDER BY customerName)
FROM
    employees
        INNER JOIN
    customers ON customers.salesRepEmployeeNumber = employeeNumber
GROUP BY employeeNumber
ORDER BY firstName , lastname;

The result set is much easier to read.

Using MySQL GROUP_CONCAT() with CONCAT_WS() function example

Sometimes, the GROUP_CONCAT the function can be combined with the CONCAT_WS function to make the result of the query more useful.

For example, to make a list of semicolon-separated values of customers:

• First, you concatenate the last name and first name of each customer’s contact using the CONCAT_WS() function. The result is the contact’s full name.
• Then, you use the GROUP_CONCAT() function to make the list.

The following query makes a list of semicolon-separated values of customers.

SELECT 
    GROUP_CONCAT(
       CONCAT_WS(', ', contactLastName, contactFirstName)
       SEPARATOR ';')
FROM
    customers;

Note that GROUP_CONCAT() function concatenates string values in different rows while the CONCAT_WS() or CONCAT()function concatenates two or more string values in different columns.

MySQL GROUP_CONCAT function: common mistakes

The GROUP_CONCAT() the function returns a single string, not a list of values. It means you cannot use the result of the GROUP_CONCAT() function for IN operator e.g., within a subquery.

For example, the GROUP_CONCAT() the function returns the result of values:1 2, and 3 as the ‘1,2,3’ string.

If you supply this result to the IN operator, the query is not working. Therefore, the query may not return any result. For example, the following query will not work as desired.

Because the IN the operator accepts a list of values e.g., (1,2,3) not a string that consists of a list of values (‘1,2,3’). As a result, the following query will not work as expected.

SELECT 
    id, name
FROM
    table_name
WHERE
    id IN GROUP_CONCAT(id);

Because the GROUP_CONCAT the function is an aggregate function, to sort the values, you must use the ORDER BY clause inside the function, not in the ORDER BY in the SELECT statement.

The following example demonstrates the incorrect usage of the ORDER BY a clause in the context of using the GROUP_CONCAT function:

SELECT 
    GROUP_CONCAT(DISTINCT country
        SEPARATOR ';')
FROM
    customers
ORDER BY country;

The SELECT clause returns one string value so the ORDER BY the clause does not take any effect on this statement.

MySQL GROUP_CONCAT() function applications

There are many cases where you can apply the GROUP_CONCAT() function to produce useful results. The following list is some common examples of using the GROUP_CONCAT() function.

• Make a comma-separated user’s roles such as ‘admin, author, editor’.
• Produce the comma-separated user’s hobbies e.g., ‘design, programming, reading’.
• Create tags for blog posts, articles, or products e.g., ‘mysql, MySQL aggregate function, MySQL tutorial’.

In this tutorial, you have learned how to use the MySQL GROUP_CONCAT() function to concatenate non-NULL values of a group of strings into a single string.