leetcode-problemset/leetcode-cn/problem (Chinese)/项目员工 I [project-employees-i].html

<p>项目表&nbsp;<code>Project</code>：&nbsp;</p>

<pre>
+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| project_id  | int     |
| employee_id | int     |
+-------------+---------+
主键为 (project_id, employee_id)。
employee_id 是员工表 <code>Employee 表的外键。</code>
</pre>

<p>员工表&nbsp;<code>Employee</code>：</p>

<pre>
+------------------+---------+
| Column Name      | Type    |
+------------------+---------+
| employee_id      | int     |
| name             | varchar |
| experience_years | int     |
+------------------+---------+
主键是 employee_id。
</pre>

<p>&nbsp;</p>

<p>请写一个 SQL&nbsp;语句，查询每一个项目中员工的&nbsp;<strong>平均&nbsp;</strong>工作年限，<strong>精确到小数点后两位</strong>。</p>

<p>查询结果的格式如下：</p>

<pre>
Project 表：
+-------------+-------------+
| project_id  | employee_id |
+-------------+-------------+
| 1           | 1           |
| 1           | 2           |
| 1           | 3           |
| 2           | 1           |
| 2           | 4           |
+-------------+-------------+

Employee 表：
+-------------+--------+------------------+
| employee_id | name   | experience_years |
+-------------+--------+------------------+
| 1           | Khaled | 3                |
| 2           | Ali    | 2                |
| 3           | John   | 1                |
| 4           | Doe    | 2                |
+-------------+--------+------------------+

Result 表：
+-------------+---------------+
| project_id  | average_years |
+-------------+---------------+
| 1           | 2.00          |
| 2           | 2.50          |
+-------------+---------------+
第一个项目中，员工的平均工作年限是 (3 + 2 + 1) / 3 = 2.00；第二个项目中，员工的平均工作年限是 (3 + 2) / 2 = 2.50
</pre>