python学习第37天

一. 单表查询

sql:查询语句的完整语法

select .. from .. where .. group by .. having .. order by .. limit ..

(1)where条件的使用

功能: 对表中的数据进行筛选和过滤

语法:

	1.判断的符号:
	= > >= < <= != <> 不等于
	2.拼接条件的关键字
	and or not
	3.查询的区间范围值 between
	between  小值  and 大值 [小值,大值] 查询两者之间这个范围内所有数据
	4.查询具体某个值的范围 in
	in(1,2,3) 指定范围
	5.模糊查询 like "%" "_" 通配符
		like "%a"  匹配以a结尾的任意长度的字符串
		like "a%"  匹配以a开头的任意长度的字符串
		like "%a%" 匹配含有a字母的任意长度的字符串
		like "_a"  个数一共2个字符,必须以a结尾,前面的字符随意
		like "a__" 个数一共3个字符,必须以a开头,后面的字符随意

# (1) 单条件的查询
# 查询部门是sale的所有员工姓名:
select emp_name from employee where post = "sale";

# (2) 多条件的查询
# 部门是teacher , 收入大于10000的所有数据
select * from employee where post="teacher" and salary > 10000;

# (3) 关键字 between .. and ..
# 收入在1万到2万之间的所有员工姓名和收入
select emp_name,salary from employee where salary between 10000 and 20000;
# 收入不在1万到2万之间的所有员工姓名和收入
select emp_name,salary from employee where salary not between 10000 and 20000;

# (4) null 关键字 在查询的时候,要用is进行判定,不要用=
# 查询post_comment 是空的所有数据
select * from employee where post_comment = null;
select * from employee where post_comment = '';
select * from employee where post_comment is null;
select * from employee where post_comment is not null;

update employee set post_comment = "" where id = 1
select * from employee where post_comment = "";	

# (5) 关键字 in 在..之中 查询
# 查询收入是3000 ,4000 ,5000,8300 所有员工的姓名和收入
select emp_name,salary from employee where salary=3000 or salary=4000 or salary=5000 or salary=8300;
# 用in优化,在小括号里面具体指定某个值
select emp_name,salary from employee where salary in (3000,4000,5000,8300);
# 不在 not in ..
select emp_name,salary from employee where salary not in (3000,4000,5000,8300);

# (6) 模糊查询 like "%" "_" 通配符
# (1) "%" 通配符  以on结尾的员工名搜一下
select emp_name,age,post from employee where emp_name like "%on";
# (2) "_" 通配符 可以限定具体的长度
select emp_name,age,post from employee where emp_name like "a_e_";

# (7) concat (as 起别名)
select concat("姓名:",emp_name,"工资:",salary) as aa from employee;
# concat_ws(拼接的符号,参数1,参数2,参数3 ... )
select concat_ws(" : ",emp_name,salary) as bb from employee;
# 计算年薪 可以在mysql中使用四则运算符(+ - * /)
select concat_ws(" : ",emp_name,salary * 12) as cc from employee;

(2)group by

分组分类

group by 字段 对数据进行分类 , by后面接什么字段,select就搜索什么字段

select sex from employee group by sex;
select post from employee group by post;

# group_concat 按照分类的形式进行字段的拼接
select group_concat(emp_name),post from employee where id >5  group by post;

聚合函数

	# count 统计总数 *所有
	select count(*) from employee;
	# max 统计最大值
	select max(salary) from employee;
	# min 统计最小值
	select min(salary) from employee;
	# avg 统计平均值
	select avg(salary) from employee;
	# sum 统计总和
	select sum(salary) from employee

一般情况下 分组 + 聚合函数 配合使用

# 1. 查询部门名以及各部门的平均薪资
select post,avg(salary) from employee group by post;		
# 2. 查询部门名以及各部门的最高薪资
select post,max(salary) from employee group by post;
# 3. 查询部门名以及各部门的最低薪资
select post,min(salary) from employee group by post;
# 4. 查询公司内男员工和女员工的个数
select sex , count(*) from employee group by sex;
# 5. 查询部门名以及部门包含的所有员工名字
select group_concat(emp_name) from employee group by post
# 可以group by 两个字段,即可搜索两个字段
select  emp_name , post from employee  group by post ,emp_name ;

(3) having

数据在分类分组之后,进行二次数据过滤,一般是配合group by 使用,分组之后在过滤

# 找出各部门平均薪资, 并且大于10000以上的所有部门
select post , avg(salary) from employee group by post having  avg(salary) > 10000;

# 1.查询各岗位内包含的员工个数小于2的岗位名、查询岗位内包含员工名字、个数
select post ,group_concat(emp_name),count(*) from employee group by post having count(*) < 2

# 2.查询各岗位平均薪资小于10000的岗位名、平均工资
select post,avg(salary) from employee  group by post having avg(salary) < 10000;

# 3.查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
select post,avg(salary) from employee group by post having avg(salary) between 10000  and 20000;# between 10000 和 20000本身这个值可以取到
select post,avg(salary) from employee group by post having avg(salary) > 10000  and avg(salary) < 20000;

(4)order by

排序 , 按照什么字段进行排序

# asc  升序: 从小到大 (默认)
# desc 降序: 从大到小

(5)limit 限制查询条数 (数据分页)

limit m,n m代表从第几条数据查询,n代表查询几条, m=0 代表的是第一条

(6)(了解)可以使用正则表达式查询数据 (不推荐使用,效率不高)

select * from employee where emp_name regexp ".*on$" # .*? 问号?不识别
select * from employee where emp_name regexp "^程";
select * from employee where emp_name regexp "^程.*金";

二. 多表查询

内连接: (内联查询 inner join ) -> 两表或者多表满足条件的所有数据查询出来(两表之间共有的数据)

# 两表查询
select 字段 from 表1 inner join 表2 on 必要的关联条件
# 多表查询
select 字段 from 表1 inner join 表2 on 必要的关联条件1 inner join 表3 on 必要的关联条件2 ... 

# 基本语法 inner join on .. on后面接必要的关联条件
select * from employee inner join department on employee.dep_id = department.id;
# 用as 起别名 (推荐)
select * from employee as e inner join department as d on e.dep_id = d.id;
# as 可以省略掉
select * from employee e inner join department d on e.dep_id = d.id;

# where 默认实现的就是内联查询的效果
select * from employee , department where employee.dep_id = department.id;
select * from employee as e , department as d where e.dep_id = d.id; 外连接

外连接

#(1) 左连接(左联查询 left join) 以左表为主,右表为辅,完整查询左表所有数据,右表没有的数据补null
select * from employee left join  department on employee.dep_id = department.id;
#(2) 右链接(右联查询 right join)以右表为主,左表为辅,完整查询右表所有数据,左表没有的数据补null
select * from employee right join  department on employee.dep_id = department.id;
#(3) 全连接(全连接 union) 所有的数据都合并起来
select * from employee1 left join  department on employee1.dep_id = department.id
union
select * from employee1 right join  department on employee1.dep_id = department.id;

三.子查询

嵌套查询

(1)sql语句当中又嵌套了另外一条sql语句,用括号()包起来,表达一个整体
(2)一般应用在from 子句后面表达一张表, where子句后面表达一个条件
(3)查询速度从快到慢 : 单表查询 -> 联表查询 -> 子查询