-- 1．列出至少有一个员工的所有部门。-- 2．列出薪金比SMITH多或者相等的所有员工。-- 3．列出所有员工的姓名及其直接上级的姓名。-- 4．列出受雇日期早于其直接上级的所有员工。-- 5．列出部门名称和这些部门的员工信息同时列出那些没有员工的部门-- 6．列出所有CLERK办事员的姓名及其部门名称。-- 7．列出最低薪金大于1500的各种工作。-- 8．列出在部门SALES销售部工作的员

SELECT department FROM employees GROUP BY department HAVING COUNT(*) >= 1;

SELECT * FROM employees WHERE salary >= (SELECT salary FROM employees WHERE last_name = 'SMITH');

SELECT e1.last_name AS employee_name, e2.last_name AS manager_name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.employee_id;

SELECT * FROM employees e1 WHERE hire_date < (SELECT hire_date FROM employees e2 WHERE e1.manager_id = e2.employee_id);

SELECT d.department_name, e.employee_name FROM departments d LEFT JOIN employees e ON d.department_id = e.department_id;

SELECT e.employee_name, d.department_name FROM employees e JOIN departments d ON e.department_id = d.department_id WHERE e.job_title = 'CLERK';

SELECT DISTINCT job_title FROM employees WHERE salary > 1500;

SELECT e.employee_name FROM employees e JOIN departments d ON e.department_id = d.department_id WHERE d.department_name = 'SALES';

SELECT * FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);

SELECT * FROM employees WHERE job_title = (SELECT job_title FROM employees WHERE last_name = 'SCOTT')