Mysql分组排序求topN

表结构

按照grp分组，按照num排序，每组取top3，输出结果如下：

源代码

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SELECT * FROM score AS t3
WHERE (
    SELECT COUNT(*) FROM score AS t1
    LEFT JOIN score AS t2
    ON t1.grp = t2.grp AND t1.num < t2.num

) < 3
ORDER BY t3.grp ASC,num DESC;