5月10日 python学习总结 单表查询 和 多表连接查询

一、 单表查询 　　

　　一 语法

select distinct 查询字段1，查询字段2，。。。 from 表名
where 分组之前的过滤条件
group by 分组依据
having 分组之后的过滤条件
order by 排序字段
limit 显示的条数;

伪代码 模仿执行过程

二 where过滤

select id,name from db39.emp where id >= 3 and id <= 6
select * from db39.emp where id between 3 and 6;

select * from emp where salary = 20000 or salary = 18000 or salary = 17000;
select * from emp where salary in (20000,18000,17000);

要求：查询员工姓名中包含i字母的员工姓名与其薪资
select name,salary from db39.emp where name like '%i%'

要求：查询员工姓名是由四个字符组成的的员工姓名与其薪资
select name,salary from db39.emp where name like '____';
select name,salary from db39.emp where char_length(name) = 4;

select * from db39.emp where id not between 3 and 6;
select * from emp where salary not in (20000,18000,17000);

要求：查询岗位描述为空的员工名与岗位名
select name,post from db39.emp where post_comment is NULL;
select name,post from db39.emp where post_comment is not NULL;

三 group by分组

#设置sql_mode为only_full_group_by，意味着以后但凡分组，只能取到分组的依据
mysql> set global sql_mode="strict_trans_tables,only_full_group_by";

#每个部门的最高工资
select post,max(salary) from emp group by post;
select post,min(salary) from emp group by post;
select post,avg(salary) from emp group by post;
select post,sum(salary) from emp group by post;
select post,count(id) from emp group by post;

#group_concat（分组之后用）
select post,group_concat(name) from emp group by post;
select post,group_concat(name,"_SB") from emp group by post;
select post,group_concat(name,": ",salary) from emp group by post;
select post,group_concat(salary) from emp group by post;

# 补充concat（不分组时用）
select name as 姓名,salary as 薪资 from emp;

select concat("NAME: ",name) as 姓名,concat("SAL: ",salary) as 薪资 from emp;

# 补充as语法
mysql> select emp.id,emp.name from emp as t1; # 报错
mysql> select t1.id,t1.name from emp as t1;

# 查询四则运算
select name,salary*12 as annual_salary from emp;

分组练习

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 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) from emp group by post;

3. 查询公司内男员工和女员工的个数
select sex,count(id) from emp 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 group by post;
5. 查询岗位名以及各岗位的最高薪资
6. 查询岗位名以及各岗位的最低薪资
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 sex,avg(salary) from emp group by sex;

8、统计各部门年龄在30岁以上的员工平均工资
select post,avg(salary) from emp where age >= 30 group by post;

四 having过滤

having的语法格式与where一模一样，只不过having是在分组之后进行的进一步过滤
即where不能用聚合函数，而having是可以用聚合函数，这也是他们俩最大的区别

1、统计各部门年龄在30岁以上的员工平均工资，并且保留平均工资大于10000的部门
select post,avg(salary) from emp
where age >= 30
group by post
having avg(salary) > 10000;

#强调：having必须在group by后面使用
select * from emp
having avg(salary) > 10000;

五 distinct去重

select distinct post,avg(salary) from emp
where age >= 30
group by post
having avg(salary) > 10000;

六 order by 排序

select * from emp order by salary asc; #默认升序排
select * from emp order by salary desc; #降序排

select * from emp order by age desc; #降序排

select * from emp order by age desc,salary asc; #先按照age降序排，再按照薪资升序排

# 统计各部门年龄在10岁以上的员工平均工资，并且保留平均工资大于1000的部门，
然后对平均工资进行排序

select post,avg(salary) from emp
where age > 10
group by post
having avg(salary) > 1000
order by avg(salary)
;

七 limit 限制显示条数

select * from emp limit 3;

select * from emp order by salary desc limit 1;

# 分页显示
select * from emp limit 0,5;
select * from emp limit 5,5;

八 正则表达式

select * from emp where name regexp '^jin.*(n|g)$';

二、 多表连接查询

1、内连接：把两张表有对应关系的记录连接成一张虚拟表

select * from emp inner join dep on emp.dep_id = dep.id;

#应用：
select * from emp,dep where emp.dep_id = dep.id and dep.name = "技术"; # 不要用where做连表的活

select * from emp inner join dep on emp.dep_id = dep.id
where dep.name = "技术"
;

2、左连接：在内连接的基础上，保留左边没有对应关系的记录

select * from emp left join dep on emp.dep_id = dep.id;

3、右连接：在内连接的基础上，保留右边没有对应关系的记录

select * from emp right join dep on emp.dep_id = dep.id;

4、全连接：在内连接的基础上，保留左、右边没有对应关系的记录

　　　　 full join  但是mysql没有full    解决方案如下：

select * from emp left join dep on emp.dep_id = dep.id
union
select * from emp right join dep on emp.dep_id = dep.id;

　　

　　补充：多表连接可以不断地与虚拟表连接

查找各部门最高工资
select t1.* from emp as t1
inner join
(select post,max(salary) as ms from emp group by post) as t2
on t1.post = t2.post
where t1.salary = t2.ms
;