MySQL has useful extension to the GROUP BY operation: function GROUP_CONCAT:
GROUP_CONCAT(expr) – This function returns a string result with the concatenated non-NULL values from a group.
When is GROUP_CONCAT useful? Usually while working with Support customers I recommend it when you have aggregation of many-to-many info. It makes the view simpler and more beautiful and it doesn’t need much effort to make it work.
In the Percona eBook, "The power of MySQL’s GROUP_CONCAT,” authors Vadim Tkachenko and Michael Rikmas provide real-world examples of GROUP_CONCAT in action.