What is the best way to handle a join / group_concat query in MySQL

I am looking for some feedback regarding the best way to handle the following (simplified for this example) situation. I have 3 tables: tasks, assignees, and users ---tasks---- id, task, completed ---assignees---- user_id, task_id ---users--- id, name I need to select tasks with concatenated user names of assignees, like so tasks.task | assigned_names.names --------------------------------------- "My task" | "Joe Schmoe, Bob Mcbart, Sally McGuire" Here is what I have currently SELECT tasks.task, assigned_names.names FROM tasks LEFT JOIN ( SELECT GROUP_CONCAT( users.name SEPARATOR ',' ) AS names, assignees.task_id FROM assignees LEFT JOIN users ON users.id = assignees.user_id GROUP BY assignees.task_id ) assigned_names ON assigned_names.task_id = tasks.id WHERE tasks.completed IS NULL My main concern is that the assignees table has hundreds of thousands of records, and doing a group on the whole table every time seems like not a good idea. Is there a better strategy? Also, I would be happy to rephrase my question title if someone could suggest better language. Thanks!