SQL 多表查询练习：同种工作不同部门员工信息和部门详细信息查询

a)查询从事同一种工作但不属于同一部门的员工信息:\n\nSELECT e1.employee_id, e1.employee_name, d1.department_name, e1.job_title\nFROM employees e1\nJOIN departments d1 ON e1.department_id = d1.department_id\nJOIN employees e2 ON e1.job_title = e2.job_title\nWHERE e1.department_id <> e2.department_id;\n\nb)查询各个部门的详细信息以及部门人数、部门平均工资:\n\nSELECT d.department_id, d.department_name, COUNT(e.employee_id) AS department_size, AVG(e.salary) AS average_salary\nFROM departments d\nJOIN employees e ON d.department_id = e.department_id\nGROUP BY d.department_id, d.department_name;