摘要：
一、首先说一下我的需求,在数据表T_CERTI_EXELAW中求certicode字段前三位出现次数最多的三位（期望结果是207），数据表如下： 期望结果是207二、两种sql语句比较（oracle数据库）方法一：常规嵌套(耗时：0.031seconds)SELECT NAME FROM (SELECT HEAD AS NAME, COUNT(HEAD) AS TIMES FROM (SELECT SUBSTR(M.CERTICODE, 0, 3) AS HEAD FROM T_CERTI_EXELAW M WHERE M.MAKEDEPART = ... 阅读全文
