统计各个部门平均薪资

drop table if exists  `staff_tb` ; 
CREATE TABLE `staff_tb` (
`staff_id` int(11) NOT NULL,
`staff_name` varchar(16) NOT NULL,
`staff_gender` char(8) NOT NULL,
`post` varchar(11) NOT NULL,
`department` varchar(16) NOT NULL,
PRIMARY KEY (`staff_id`));
INSERT INTO staff_tb VALUES(1,'Angus','male','Financial','dep1'); 
INSERT INTO staff_tb VALUES(2,'Cathy','female','Director','dep1'); 
INSERT INTO staff_tb VALUES(3,'Aldis','female','Director','dep2'); 
INSERT INTO staff_tb VALUES(4,'Lawson','male','Engineer','dep1'); 
INSERT INTO staff_tb VALUES(5,'Carl','male','Engineer','dep2'); 
INSERT INTO staff_tb VALUES(6,'Ben','male','Engineer','dep1'); 
INSERT INTO staff_tb VALUES(7,'Rose','female','Financial','dep2'); 

drop table if exists  `salary_tb` ; 
CREATE TABLE `salary_tb` (
`salary_id` int(11) NOT NULL,
`staff_id` int(11) NOT NULL,
`normal_salary` int(11) NOT NULL,
`dock_salary` int(11) NOT NULL,
PRIMARY KEY (`salary_id`));
INSERT INTO salary_tb VALUES(10,1,12000,2500); 
INSERT INTO salary_tb VALUES(11,2,11000,2200); 
INSERT INTO salary_tb VALUES(12,3,9000,1800); 
INSERT INTO salary_tb VALUES(13,4,10500,1900); 
INSERT INTO salary_tb VALUES(14,5,13500,2100); 
INSERT INTO salary_tb VALUES(15,6,7500,1000); 
INSERT INTO salary_tb VALUES(16,7,50000,5000);

department|avg_salary
dep2|9300.000
dep1|8350.000