how do i get rows for each month which contain the store with the highest number of purchases?
SELECT t1.*
FROM purchase t1
JOIN ( SELECT t2.`month`, MAX(t2.numPurchases) numPurchases
       FROM purchase t2
       GROUP BY t2.`month` ) t3 
    ON t1.`month` = t3.`month`
   AND t1.numPurchases = t3.numPurchases