SQL练习题13：获取所有部门中当前(dept_emp.to_date = '9999-01-01')员工当前(salaries.to_date='9999-01-01')薪水最高的相关信息，给出dept_no, emp_no以及其对应的salary，按照部门升序排列。

PRIMARY KEY (`emp_no`,`from_date`));

INSERT INTO dept_emp VALUES(10001,‘d001‘,‘1986-06-26‘,‘9999-01-01‘);
INSERT INTO dept_emp VALUES(10002,‘d001‘,‘1996-08-03‘,‘9999-01-01‘);
INSERT INTO dept_emp VALUES(10003,‘d001‘,‘1996-08-03‘,‘1997-08-03‘);

INSERT INTO salaries VALUES(10001,90000,‘1986-06-26‘,‘1987-06-26‘);
INSERT INTO salaries VALUES(10001,88958,‘2002-06-22‘,‘9999-01-01‘);
INSERT INTO salaries VALUES(10002,72527,‘1996-08-03‘,‘1997-08-03‘);
INSERT INTO salaries VALUES(10002,72527,‘2000-08-02‘,‘2001-08-02‘);
INSERT INTO salaries VALUES(10002,72527,‘2001-08-02‘,‘9999-01-01‘);
INSERT INTO salaries VALUES(10003,90000,‘1996-08-03‘,‘1997-08-03‘);

则输出

dept_no	emp_no	salary
d001	10001	88958

SELECT a.dept_no, a.emp_no, MAX(b.salary)
FROM dept_emp AS a INNER JOIN salaries AS b
ON a.emp_no = b.emp_no
WHERE a.to_date = ‘9999-01-01‘
AND b.to_date = ‘9999-01-01‘
GROUP BY a.dept_no
ORDER BY a.dept_no ASC