This gets the song_id and the number of sales of the three most purchased songs:
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SELECT `music_purchased`.`song_id`, COUNT(`song_id`) AS 'sales' FROM `music_purchased` GROUP BY `song_id` ORDER BY COUNT(`song_id`) DESC LIMIT 0,3;

this gets the name of the song knowing the song_id:
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SELECT `song_title` FROM `music_table` WHERE `song_id`=ID;