GROUP_CONCAT is cool, but I stopped using it in MySQL 5.6. Unfortunately, it has a maximum_length setting which by default limits the return value to 1024 characters. It's the kind of bug that can go unnoticed for months, or only be evident with table growth. I wish it didn't have a limit (or that you could set no limit on it) and did not require fiddling with the DB settings. So beware! :)
I'm not sure about newer versions though.
Good warning sign to take of.
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