sql，SELECT employee_id, first_name, last_name，FROM employees，WHERE department_id = (SELECT department_id FROM departments WHERE department_name = 'Sales');，`，，这个例子中，子查询 (SELECT department_id FROM departments WHERE department_name = 'Sales') 返回销售部门的 department_id，然后主查询根据这个 department_id` 筛选出员工信息。