顺序：

where——〉group by——〉having——〉order by.

示例1：

SELECT WORKDEPT, MAX(SALARY) AS MAXIMUM, MIN(SALARY) AS MINIMUM
FROM EMPLOYEE
GROUP BY WORKDEPT
HAVING COUNT(*) > 4
ORDER BY WORKDEPT

功能：查询雇员数超过4的部门的最高和最低薪水。

示例2：

SELECT WORKDEPT, EDLEVEL, MAX(SALARY) AS MAXIMUM
FROM EMPLOYEE
WHERE HIREDATE > '1979-01-01'
GROUP BY WORKDEPT, EDLEVEL
ORDER BY WORKDEPT, EDLEVEL
功能：按照 WORKDEPT, EDLEVEL分组，计算每个组中雇员在'1979-01-01'之后的最高薪水和最低薪水，

结果按照WORKDEPT, EDLEVEL排序显示。

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分组函数使用顺序

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