需求是求以下表中每一种分类销量最多的三种商品：
方法1：
SELECT TBL.ID,TBL.PRODUCTNAME,TBL.TYPENAME,TBL.SALECOUNT
FROM PRODUCT TBL
LEFT JOIN PRODUCT L_TBL ON TBL.TYPENAME = L_TBL.TYPENAME AND TBL.SALECOUNT< L_TBL.SALECOUNT
GROUP BY TBL.ID,TBL.PRODUCTNAME,TBL.TYPENAME,TBL.SALECOUNT
HAVING COUNT(L_TBL.ID)< 3
ORDER BY TBL.TYPENAME,TBL.SALECOUNT DESC
方法2：
SELECT TBL.ID,TBL.PRODUCTNAME,TBL.TYPENAME,TBL.SALECOUNT
FROM PRODUCT TBL
WHERE 3>(SELECT COUNT(*) FROM PRODUCT WHERE TYPENAME =TBL.TYPENAME AND SALECOUNT>TBL.SALECOUNT)
ORDER BY TBL.TYPENAME,TBL.SALECOUNT DESC
查询结果：
