mysql的三表查询的例子

三表查询是指在同一条SQL语句中查询三个或更多的表，并将它们连接起来。下面是一个MySQL的三表查询例子：

有三张表：

employees:
+----+-------+--------+----------+
| id | name  | gender | hiredate |
+----+-------+--------+----------+
|  1 | Tom   | M      | 2015-07-01 |
|  2 | Jerry | F      | 2016-09-12 |
|  3 | Bob   | M      | 2017-02-15 |
+----+-------+--------+----------+

departments:
+----+------------+
| id | name       |
+----+------------+
|  1 | Sales      |
|  2 | Marketing  |
|  3 | HR         |
+----+------------+

employee_departments:
+----+-------------+---------------+
| id | employee_id | department_id |
+----+-------------+---------------+
|  1 |           1 |             1 |
|  2 |           1 |             2 |
|  3 |           2 |             2 |
|  4 |           3 |             3 |
+----+-------------+---------------+

三表查询语句如下：

SELECT employees.name, departments.name
FROM employees
JOIN employee_departments ON employees.id = employee_departments.employee_id
JOIN departments ON employee_departments.department_id = departments.id;

查询结果如下：

+-------+-----------+
| name  | name      |
+-------+-----------+
| Tom   | Sales     |
| Tom   | Marketing |
| Jerry | Marketing |
| Bob   | HR        |
+-------+-----------+

这个例子中，我们想要查询每个员工所在的部门。我们使用了 `JOIN` 来连接三张表，第一个 `JOIN` 连接了 employees 和 employee_departments 表，第二个 `JOIN` 连接了 employee_departments 和 departments 表。最终结果是返回了每个员工的名字和他们所在的部门。