查找第二大数字

max()


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SELECT max(sid) FROM `tbl_test` where sid < (SELECT max(sid) FROM `tbl_test`);
SELECT max(sid) FROM `tbl_test` where sid != (SELECT max(sid) FROM `tbl_test`);

order by、group by


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SELECT min(sid) FROM (SELECT sid FROM `tbl_test` order BY sid DESC limit 2) as tmp;
SELECT min(sid) FROM (SELECT sid FROM `tbl_test` group BY sid DESC limit 2) as tmp;