查询出公司员工的 last_name,department_name, city（三 表关联查询，用两种方法来写）

查询公司员工的姓氏(last_name)、部门名(department_name)以及所在城市(city)，通常需要从三个相关的表（假设分别为 employees, departments 和 cities）进行联接。这里有两种常见的 SQL 方法来实现这种关联查询：

**方法一：INNER JOIN (内连接)**

SELECT e.last_name, d.department_name, c.city
FROM employees AS e
INNER JOIN departments AS d ON e.department_id = d.department_id
INNER JOIN cities AS c ON d.city_id = c.city_id;

在这个查询中，我们首先通过 `employees` 表和 `departments` 表的 `department_id` 进行匹配，然后通过 `departments` 表和 `cities` 表的 `city_id` 进行匹配。

**方法二：DEPARTMENT_ID IN (子查询) 或 EXISTS (嵌套查询) **

-- 方法一：使用子查询
SELECT e.last_name, d.department_name, c.city
FROM employees AS e
WHERE e.department_id IN (
    SELECT department_id FROM departments WHERE city_id IN (SELECT city_id FROM cities)
);

-- 方法二：使用 EXISTS
SELECT e.last_name, d.department_name, c.city
FROM employees AS e
JOIN departments AS d ON e.department_id = d.department_id
JOIN cities AS c ON d.city_id = c.city_id
WHERE EXISTS (SELECT * FROM cities WHERE cities.city_id = d.city_id);

在这两种方法中，都会返回那些在所有相关表中有对应记录的员工的姓氏、部门名称和城市信息。