mysql之多表查询的其他查询

1，临时表查询

（1）需求：查询高于本部门平均工资的人员

select * from person as p, (select dept_id, avg(salary) as '平均工资' from person GROUP BY dept_id) as ptable where p.dept_id = ptable.dept_id and p.salary > ptable.`平均工资`;

（2）需求：查询高于本部门平均工资的人员，显示部门名

第一步查询出每个部门的平均工资，并显示部门名

select * from (select dept_id, avg(salary) as '平均工资' from person GROUP BY dept_id) as p, dept as pd where p.dept_id = pd.did;

第二步再对每个人的工资和第一步查询出来的结果集（让其作为临时表）的平均工资比较

select * from person as p, (select * from (select dept_id, avg(salary) as '平均工资' from person GROUP BY dept_id) as pa, dept as pd where pa.dept_id = pd.did) as ptable where p.dept_id = ptable.dept_id and p.salary > ptable.`平均工资`;

2.判断查询 IF关键字

（1）需求：根据工资高低，将人员划分为两个级别，分别为高端人群和低端人群。显示效果：姓名、年龄、性别、工资、级别

 -- IF关键字可以带三个参数，参数1条件,参数2为参数1条件成立时使用的，参数3为参数1条件不成立时使用
select p.name, p.age, p.sex, p.salary, IF(p.salary > 10000, '高端人群', '低端人群') as '级别' from person as p;

（2）需求：根据工资高低统计每个部门人员收入情况，划分为富人、小资、平民、屌丝四个级别，要求统计四个级别分别有多少人

select dname,
    sum(case WHEN person.salary > 10000 THEN 1 else 0 END) as '富人',
    sum(case WHEN person.salary BETWEEN 5000 and 10000 THEN 1 else 0 END) as '小资',
    sum(case WHEN person.salary BETWEEN 3000 and 5000 THEN 1 else 0 END) as '平民',
    sum(case WHEN person.salary < 3000 THEN 1 else 0 END) as '屌丝'
from dept, person where dept.did = person.dept_id GROUP BY dept_id;