sql - Oracle SQL 为什么查询返回多个结果？

Question

我被一个查询困住了，我无法让它正常工作。

这是我需要做的：

编写一个查询，该查询将返回雇员表中按部门分组的最高和最低平均工资。

这是我做的：

SELECT
(SELECT AVG(MIN(salary)) FROM employees GROUP BY department_id) As "Minimum Average salary",
(SELECT AVG(MAX(salary)) FROM employees GROUP BY department_id) As "Maximum Average Salary"
FROM EMPLOYEES

但它不断给我返回超过 1 行的结果。我不能使用 LIMIT 2，如果我使用 LIMIT 查询会出错。

我也尝试了以下查询，但出现错误：缺少表达式。

SELECT
AVG(SELECT MIN(salary) FROM employees GROUP BY department_id)) As "Minimum Average salary",
AVG((SELECT MAX(salary) FROM employees GROUP BY department_id)) As "Maximum Average Salary"
FROM EMPLOYEES

Answer 1

不需要内联视图 - 这有效：

select
  min(avg(salary)),
  max(avg(salary))
from
 employees
group by
  dept_id;

http://sqlfiddle.com/#!4/eff67/3

Answer 2

使用子查询检索所有平均工资。然后外部查询可以获取最小值和最大值：

select  min(AvgSalary)
,       max(AvgSalary)
from    (
        select  avg(salary) as AvgSalary
        from    employees
        group by
                department_id
        ) SubQueryAlias

Answer 3

首先，它返回与 EMPLOYEES 表中的所有记录一样多的原因很简单；因为 group by 在您的内部子查询中，它不适用于您的主查询。实际上，您的子查询是按表中的每一行执行的，并且每行返回结果。我认为这是您需要的正确查询：

SELECT AVG(min_salary) As "Minimum Average salary", AVG(max_salary) As "Maximum Average Salary"
FROM
(SELECT MIN(salary) As min_salary, MAX(salary) AS max_salary
 FROM EMPLOYEES
 GROUP BY department_id
)

Answer 4

select MAX(salary),min(salary), avg(salary), department_id from employees  group by department_id