Was bedeutet dieser Code?

update instance inner join (select group.id as group_id, (select message.id from message inner join thread on thread.id = message.thread_id where location_id = @location_id and language_id = @language_id and concat(group_key, '.') like concat(group.`key`, '.%') order by message.created desc limit 1) as last_message_id, (select count(*) from thread where location_id = @location_id and language_id = @language_id and concat(group_key, '.') like concat(group.`key`, '.%')) as thread_count, (select if(sum(thread.message_count) is null, 0, sum(thread.message_count)) from thread where location_id = @location_id and language_id = @language_id and concat(group_key, '.') like concat(group.`key`, '.%')) as message_count from group where @group_key like concat(`key`, '.%')) as statistics on statistics.group_id = instance.group_id set instance.message_id = statistics.last_message_id, instance.thread_count = statistics.thread_count, instance.message_count = statistics.message_count where instance.location_id = @location_id and instance.language_id = @language_id;