python学习笔记 day44 其他查询

还是person表和dept表（跟之前用的一样）

 1. 其他查询

1.1 临时表查询

题目1：查询高于本部门平均工资的人员：

select * from person as p1,(select dept_id,avg(salary)as salary from person group by dept_id) as p2 where p1.dept_id=p2.dept_id and p1.salary>p2.salary;

运行结果：

其实看题也应该知道是需要用到两张表，因为需要显示高于部门平均工资的所有人员信息，需要用到person表，另外一张就是基于dept_id进行分组的平均工资表；

1.2 判断查询--IF语句

首先看一下IF语句的用法，，IF语句查询结果可以作为该表的一个字段信息：

select name,age,if(age<25,"小鲜肉","老腊肉") as "age_description",if(salary>19000,"高端人群","低端人群")as class from person;  # if(if_condition,True_result,False_result)三目运算符，条件为真，返回True_result,反之 False_result;

运行结果：

题目2：根据工资将人员分为高端人群和低端人群，显示效果： 姓名，年龄，薪资，级别；

select p1.name,p1.age,p1.salary,if(p1.salary>p2.salary,"高端人士","低端人士")as class from person as p1,(select dept_id,avg(salary)as salary from person group by dept_id) as p2 where p1.dept_id=p2.dept_id;

运行结果：

 题目3.  根据工资高低，统计每个部门人员的收入情况，划分为富人，小资，平民，屌丝，四个级别 并且要求统计四个级别分别有多少人；

效果图：

先来介绍一下 多条件选择 select case when then...

语法一：select case "字段名" when "字段的条件 或者其中的一个值" then... when "字段的其他条件或者另外一个值" then... else ... end as "起一个别名作为字段" from “表”;

select *,case sex when "女" then "是女生啊"
  when "男" then "是男生啊"
  else "其他类别" end as "sex_description"
  from person;
  

运行结果：

语法二：select case when 字段条件 then .... when ...then... else...end as "起一个别名作为字段" from "表"；

select name,age,salary,sex,case when salary<20000 then "屌丝"
  when salary between 20000 and 30000 then "小资"
  else "富人" END as "salary_description"   # 给case...when ...then ...else ...end 起一个别名作为表person的一个字段
  from person

 运行结果：

select dept.name,
sum(case when salary<19000 then 1 else 0 end )as "富人",
sum(case when salary between 19000 and 24000 then 1 else 0 end) as "平民",
sum(case when salary between 24000 and 19000 then 1 else 0 end) as "小资",
sum(case when salary>29000 then 1 else 0 end) as "富人"
from dept,person where dept.did=person.dept_id group by dept.did;

运行结果：

 2. SQL逻辑查询语句的执行顺序；

伪代码：

select Distinct <select_list>
from <left_table>
<join_type> join <right_table>
ON <join_condition>
where <where_condition>
Group by <group_by_list>
Having <having_condition>
Order by <order_by_condition>
limit <limit_condition>

执行顺序：

from 表 ---> on 关联表的条件---->join 连接表----- where 表查询的条件 ---group by 分组原则----> having 分组条件--->select 查询表 ---->order by 排序原则----->limit 分页显示；