公用表表达式（CTE）

相当于可以复用的子查询

1. 普通公用表表达式

   # 查询员工表中所有员工的部门详细信息
   WITH cte_emp
   AS
   (SELECT DISTINCT department_id FROM employees);
   
   SELECT *
   FROM departments d JOIN cte_emp e
   ON d.department_id = e.department_id;
   

2. 递归公用表表达式

   # 找出公司中所有的下下属
   # 找到第一代管理者（老大）
   WITH RECURSIVE cte
   AS
   (SELECT employee_id, last_name, manager_id, 1 AS n
   FROM employees
   WHERE employee_id = 100;
   UNION ALL
   SELECT a.employee_id, a.last_name, a.manager_id, n+1
   FROM employees AS a JOIN cte
   ON a.manager_id = cte.employee_id
   )
   
   SELECT employee_id, last_name
   FROM cte
   WHERE n >= 3;