作业 5/6

作业

1.整理今日内容
2.完成下列分组查询练习题(以课上建表代码为参考)
	1. 查询岗位名以及岗位包含的所有员工名字
        select post,group_concat(name) from emp_utf8 group by post;
	2. 查询岗位名以及各岗位内包含的员工个数
        select post,count(id) as '员工个数' from emp_utf8 group by post;
	3. 查询公司内男员工和女员工的个数
    	select sex,count(sex) as '个数' from emp_utf8 group by sex;
	4. 查询岗位名以及各岗位的平均薪资
    	select post,avg(salary) from emp_utf8 group by post；
	5. 查询岗位名以及各岗位的最高薪资
    	select post,max(salary) from emp_utf8 group by post;
	6. 查询岗位名以及各岗位的最低薪资
    	select post,min(salary) from emp_utf8 group by post;
	7. 查询男员工与男员工的平均薪资，女员工与女员工的平均薪资
    	select group_concat(name),avg(salary) as '平均薪资' from emp_utf8 where sex = 'male';
        select group_concat(name),avg(salary) as '平均薪资' from emp_utf8 where sex = 'female';

3.练习拼表操作并理解其意义
create table wl1(id int primary key auto_increment,name char(10) not null) default charset utf8;
create table wl2(id int primary key auto_increment,age int not null) default charset utf8;
insert into wl1(name) values('a');
insert into wl1(name) values('b');
insert into wl1(name) values('c');
insert into wl2(age) values(1);
insert into wl2(age) values(2);
insert into wl2 values(5,3);
select * from wl1 inner join wl2 on wl1.id = wl2.id;
select * from wl1 left join wl2 on wl1.id = wl2.id;
select * from wl1 right join wl2 on wl1.id = wl2.id;
select * from wl1 left join wl2 on wl1.id = wl2.id
union
select * from wl1 right join wl2 on wl1.id = wl2.id;
4.理解子查询思路体会其意义
首先，得到第一步产生的表，再在这个基础上继续筛选。