[mysql] MySQL and GROUP_CONCAT() maximum length

I'm using GROUP_CONCAT() in a MySQL query to convert multiple rows into a single string. However, the maximum length of the result of this function is 1024 characters.

I'm very well aware that I can change the param group_concat_max_len to increase this limit:

SET SESSION group_concat_max_len = 1000000;

However, on the server I'm using, I can't change any param. Not by using the preceding query and not by editing any configuration file.

So my question is: Is there any other way to get the output of a multiple row query into a single string?

You can try this

SET GLOBAL group_concat_max_len = 1000000;

SET SESSION group_concat_max_len = 1000000;

is a temporary, session-scope, setting. It only applies to the current session You should use it like this.

SET SESSION group_concat_max_len = 1000000;
select group_concat(column) from table group by column

You can do this even in sharing hosting, but when you use an other session, you need to repeat the SET SESSION command.

Include this setting in xampp my.ini configuration file:

[mysqld]
group_concat_max_len = 1000000

Then restart xampp mysql

The short answer: the setting needs to be setup when the connection to the MySQL server is established. For example, if using MYSQLi / PHP, it will look something like this:

$ myConn = mysqli_init(); 
$ myConn->options(MYSQLI_INIT_COMMAND, 'SET SESSION group_concat_max_len = 1000000');

Therefore, if you are using a home-brewed framework, well, you need to look for the place in the code when the connection is establish and provide a sensible value.

I am still using Codeigniter 3 on 2020, so in this framework, the code to add is in the application/system/database/drivers/mysqli/mysqli_driver.php, the function is named db_connect();

public function db_connect($persistent = FALSE)
    {
        // Do we have a socket path?
        if ($this->hostname[0] === '/')
        {
            $hostname = NULL;
            $port = NULL;
            $socket = $this->hostname;
        }
        else
        {
            $hostname = ($persistent === TRUE)
                ? 'p:'.$this->hostname : $this->hostname;
            $port = empty($this->port) ? NULL : $this->port;
            $socket = NULL;
        }

        $client_flags = ($this->compress === TRUE) ? MYSQLI_CLIENT_COMPRESS : 0;
        $this->_mysqli = mysqli_init();

        $this->_mysqli->options(MYSQLI_OPT_CONNECT_TIMEOUT, 10);
        $this->_mysqli->options(MYSQLI_INIT_COMMAND, 'SET SESSION group_concat_max_len = 1000000');

...
    }

The correct parameter to set the maximum length is:

SET @@group_concat_max_len = value_numeric;

value_numeric must be > 1024; by default the group_concat_max_len value is 1024.

The correct syntax is mysql> SET @@global.group_concat_max_len = integer;
If you do not have the privileges to do this on the server where your database resides then use a query like:
mySQL="SET @@session.group_concat_max_len = 10000;"or a different value.
Next line:
SET objRS = objConn.Execute(mySQL)  your variables may be different.
then
mySQL="SELECT GROUP_CONCAT(......);" etc
I use the last version since I do not have the privileges to change the default value of 1024 globally (using cPanel).
Hope this helps.