MySQL has useful extention to the GROUP BY operation: function GROUP_CONCAT: GROUP_CONCAT(expr) – This function returns a string result with the concatenated non-NULL values… remove loop from PHP to MySQL server side. Also it can be handy to use result concatenated string as part of IN…

[...] GROUP_CONCAT useful GROUP BY extension: 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. [...]