Mysql--查询语句总结

查询各个部门的最高工资及姓名，其中薪资字段是字符串类型：

优化前：

SELECT
*
FROM
(SELECT
a.`deptno`, a.`sal`, a.`ename`
FROM
emp a
ORDER BY CONVERT(a.`sal`, SIGNED) DESC) aa
GROUP BY aa.deptno;

优化后：

SELECT
aa.`ename`, aa.`sal`, aa.`deptno`
FROM
emp aa,
(SELECT
a.`deptno`, MAX(CONVERT(a.`sal`, SIGNED)) max_sal
FROM
emp a
GROUP BY a.`deptno`) bb
WHERE aa.`deptno` = bb.deptno
AND aa.`sal` = bb.max_sal;