写几个有难度的sql语句

当然，我可以为您提供一些有难度的 SQL 查询语句示例：

1. 查询每个部门的平均工资，并按照平均工资降序排列：

   SELECT department_id, AVG(salary) as avg_salary
   FROM employees
   GROUP BY department_id
   ORDER BY avg_salary DESC;

2. 查找至少在两个部门工作过的员工：

   SELECT employee_id, COUNT(DISTINCT department_id) as num_departments
   FROM employee_departments
   GROUP BY employee_id
   HAVING num_departments >= 2;

3. 找出有连续三天以上缺勤的员工：

   SELECT employee_id, absence_date
   FROM absences
   WHERE absence_date NOT IN (
       SELECT DATE_ADD(a.absence_date, INTERVAL 1 DAY)
       FROM absences AS a
       WHERE a.employee_id = absences.employee_id
       AND a.absence_date = DATE_SUB(absences.absence_date, INTERVAL 1 DAY)
   );

这些查询都涉及到一些复杂的概念和子查询，希望对您有所帮助！如果您有其他问题，请随时提问。