oracle各部门平均工资

Question

我有以下代码，它可以找到每个部门的平均工资，并且运行良好。

我在将 department_name 添加到输出时遇到问题。我知道我必须加入表格，但我无法让它工作。

如有任何帮助，我们将不胜感激。感谢所有回答的人。


CREATE table dept  (department_id, department_name) AS
SELECT 1, 'IT' FROM DUAL UNION ALL
SELECT 2, 'SALES'  FROM DUAL;

CREATE TABLE employees (employee_id, manager_id, first_name, last_name, department_id, sal,
serial_number) AS
SELECT 1, NULL, 'Alice', 'Abbot', 1, 100000, 'D123' FROM DUAL UNION ALL
SELECT 2, 1, 'Beryl', 'Baron',1, 50000,'D124' FROM DUAL UNION ALL
SELECT 3, 1, 'Carol', 'Chang',1, 100000, 'A1424' FROM DUAL UNION ALL
SELECT 4, 2, 'Debra', 'Dunbar',1, 75000, 'A1425' FROM DUAL UNION ALL
SELECT 5, NULL, 'Emily', 'Eden',2, 90000, 'C1725' FROM DUAL UNION ALL
SELECT 6, 3, 'Fiona', 'Finn',1, 88500,'C1726' FROM DUAL UNION ALL
SELECT 7,5, 'Grace', 'Gelfenbein',2, 55000, 'C1727' FROM DUAL;
 
SELECT
    department_id,
    ROUND(AVG(sal), 2) AS AVERAGE_SALARY
FROM employees 
group by 
department_id;

Expected output

DEPARTMENT_ID DEPARTMENT_NAME AVERAGE_SALARY
1    IT         82700
2    SALES 72500

Answer 1

您可以使用这样的查询：

SELECT
    d.department_id,
    d.department_name,
    ROUND(AVG(e.sal), 2) AS AVERAGE_SALARY
FROM employees e inner join dept d on e.department_id = d.department_id
group by 
d.department_id, d.department_name;

举个例子：https://dbfiddle.uk/?rdbms=oracle_18&fiddle=156f4e503bc3d6bfed3009137acacb23

根据department_id加入部门的员工。

做group by的时候，把统计中没有包含的2个字段也包含进去。这将为您提供您正在查找的 2 个字段的统计信息。

你的结果将是

DEPARTMENT_ID | DEPARTMENT_NAME | AVERAGE_SALARY
------------: | :-------------- | -------------:
            2 | SALES           |          72500
            1 | IT              |          82700

Answer 2

由于每个组中只有一个 department_name，因此您可以加入表并在 department_name 上使用聚合函数：

SELECT e.department_id,
       MAX(d.department_name) AS department_name,
       ROUND(AVG(e.sal), 2) AS AVERAGE_SALARY
FROM   employees e
       INNER JOIN dept d
       ON (e.department_id = d.department_id)
GROUP BY e.department_id;

或者，您可以聚合然后加入：

SELECT e.department_id,
       d.department_name,
       e.average_salary
FROM   (
         SELECT department_id,
                ROUND(AVG(sal), 2) AS average_salary
         FROM   employees
         GROUP BY department_id
       ) e
       INNER JOIN dept d
       ON (e.department_id = d.department_id);

其中，对于您的示例数据，两者都输出：

DEPARTMENT_ID DEPARTMENT_NAME AVERAGE_SALARY

2 SALES 72500

1 IT 82700

db<>fiddle here

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