一丶语法顺序

1. select
2. from
3. where
4. group by

二丶执行顺序

1. from
2. where
3. group by
4. select

实验表准备

创建表

create table emp(
  id int not null unique auto_increment,
  name varchar(20) not null,
  sex enum('male','female') not null default 'male', #大部分是男的
  age int(3) unsigned not null default 28,
  hire_date date not null,
  post varchar(50),
  post_comment varchar(100),
  salary double(15,2),
  office int, #一个部门一个屋子
  depart_id int
);

添加表数据

#插入记录
#三个部门：教学，销售，运营
insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('jason','male',18,'20170301','张江第一帅形象代言',7300.33,401,1), #以下是教学部
('egon','male',78,'20150302','teacher',1000000.31,401,1),
('kevin','male',81,'20130305','teacher',8300,401,1),
('tank','male',73,'20140701','teacher',3500,401,1),
('owen','male',28,'20121101','teacher',2100,401,1),
('jerry','female',18,'20110211','teacher',9000,401,1),
('nick','male',18,'19000301','teacher',30000,401,1),
('sean','male',48,'20101111','teacher',10000,401,1),

('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),

('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;
#ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk

三丶where约束条件

在你刚开始接触mysql查询的时候，建议你按照查询的优先级顺序拼写出你的sql语句

# 1.查询id大于等于3小于等于6的数据
select * from emp where id>= 3 and id<= 6;
select * from emp where id between 3 and 6;

# 2.查询薪资是20000或者18000或者17000的数据
select * from emp where salary = 20000 or salary = 18000 or salary = 17000;
select * from emp where salary in (20000, 18000, 17000);

# 3.查询员工姓名中包含o字母的员工姓名和薪资
select * from emp where name like "%o%";

# 4.查询员工姓名是由四个字符组成的员工姓名与其薪资
select * from emp where name like "____";

# 5.查询id小于3或者大于6的数据
select * from emp where id<= 3 or id>= 6;
select * from emp where not id between 3 and 6;

# 6.查询薪资不在20000，18000，17000范围的数据
select * from emp where salary not in (20000,18000,17000);

# 7.查询岗位描述为空的员工名与岗位名  针对null不能用等号，只能用is
select name,post  from emp where post_comment is not NULL;

四丶group by

分组后可以使用聚合函数

# 数据分组应用场景：每个部门的平均薪资，男女比例等
# 1.按部门分组
select post from emp group by post;

# 2.获取每个部门的最高工资 
# 每个部门的最高工资()
select post,max(salary) from emp group by post;

# 每个部门的最低工资
select post,min(salary) from emp group by post;

# 每个部门的平均工资
select post,avg(salary) from emp group by post;

# 每个部门的工资总和
select post,sum(salary) from emp group by post;

# 每个部门的人数
select post,count(id) from emp group by post;

# 3.查询分组之后的部门名称和每个部门下所有的成员姓名
select post,group_concat(name) from emp group by post;

# group_concat（分组之后用）不仅可以用来显示除分组外字段还有拼接字符串的作用
select post, group_concat("教师:",name) from emp group by post;

# 4.补充concat（不分组时用）拼接字符串达到更好的显示效果 as语法使用
select concat("薪资:",salary),concat("姓名:",name) from emp where salary >= 5000;

# 补充as语法 即可以给字段起别名也可以给表起
select concat("薪资:",salary) as "薪资",concat("姓名:",name) as "姓名" from emp where salary >= 5000;

# 查询四则运算
# 查询每个人的年薪
select concat("姓名:",name) as "姓名",concat("薪资:",salary*12) as "年薪" from emp where salary >= 5000;

五丶having

having的语法格式与where一致，只不过having是在分组之后进行的过滤，即where虽然不能用聚合函数，但是having可以！

强调：having必须在group by后面使用!!!

实验表建立

select post,avg(salary) from emp2
        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;  # 报错

六丶distinct

对有重复的展示数据进行去重操作

select distinct post from emp;

七丶order by

排序

asc(默认) 升序

desc 降序

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

#先按照age降序排，在年轻相同的情况下再按照薪资升序排
select * from emp order by age desc,salary asc; 

# 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)
    ;

八丶limit

# 限制展示条数
select * from emp limit 3;
# 查询工资最高的人的详细信息
select * from emp order by salary desc limit 1;

# 分页显示
select * from emp limit 0,5;  # 第一个参数表示起始位置，第二个参数表示的是条数，不是索引位置
select * from emp limit 5,5;

九丶正则

select * from emp where name regexp '^j.*(n|y)$';   #这边的正则解读为以j开头,以y或者n结尾,中间尽可能匹配更多的字符

原文地址：https://www.cnblogs.com/suren-apan/p/11561996.html