mysql 中的单表查询

单表查询：

create table emp (
    id int primary key auto_increment,
    name varchar(16) not null ,
    sex enum('male','female')not null default 'male',
    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,
    depart_id int
    );

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)
;


where 约束条件
# 1.查询id大于等于3小于等于6的数据
select id name 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 =2000 or salary =18000or 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 模糊查询
select name,salary 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 name,salary from emp where name like '____';
select name,salary from emp where char_length(name) = 4;
# 5.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 = NULL;  # 查询为空！
select name,post from emp where post_comment is NULL;
select name,post from emp where post_comment is not NULL;

group by 约束条件
group by我们可以先从字面上来理解，GROUP表示分组，BY后面写字段名，就表示根据哪个字段进行分组，如果有用Excel比较多的话，GROUP BY比较类似Excel里面的透视表。
group by必须得配合聚合函数来用，分组之后你可以计数（COUNT），求和（SUM），求平均数（AVG）等。
常用聚合函数：求个数count 求总和 sum ，求最大值 max 求最小值 min 求平均值 avg

 1.按部门分组
select * from emp group by post;  # 分组后取出的是每个组的第一条数据
select id,name,sex from emp group by post;  # 验证
"""
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，
不应该在去取组里面的单个元素的值，那样的话分组就没有意义了，因为不分组就是对单个元素信息的随意获取
"""
set global sql_mode="strict_trans_tables,only_full_group_by";
# 重新链接客户端
select * from emp group by post;  # 报错
select id,name,sex from emp group by post;  # 报错
select post from emp group by post;  # 获取部门信息
# 强调:只要分组了，就不能够再“直接”查找到单个数据信息了，只能获取到组名


# 2.获取每个部门的最高工资
# 以组为单位统计组内数据>>>聚合查询(聚集到一起合成为一个结果)
# 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学生姓名
# 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(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;


# 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 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 即可以给字段起别名也可以给表起
select emp.id,emp.name from emp as t1; # 报错  因为表名已经被你改成了t1
select t1.id,t1.name from emp as t1;

# 查询四则运算
# 查询每个人的年薪
select name,salary*12 as annual_salary from emp;
select name,salary*12 annual_salary from emp;  # as可以省略

having 约束条件
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！
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

distinct
# 对有重复的展示数据进行去重操作
select distinct post from emp;

order by 排序约束条件
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)
    ;


limit 限制约束条件
# 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;


正则匹配约束条件
select * from emp where name regexp '^j.*(n|y)$';