2.mysql之单表查询

单表查询的语法

select * from 表名  # *所有
select distinct 字段 from  表名  #distinct 去重
           where   #条件
           group by  字段   #分组
           having   #过滤
           order by  字段  #排序
           limit  #限制条数

关键字的优先级

from
where
group by
having
select
distinct
order by
limit

简单查询

创建表
create table employee(
    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
);
#三个部门：教学，销售，运营
insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部，全都是老师
('alex','male',78,'20150302','teacher',1000000.31,401,1),
('wupeiqi','male',81,'20130305','teacher',8300,401,1),
('yuanhao','male',73,'20140701','teacher',3500,401,1),
('liwenzhou','male',28,'20121101','teacher',2100,401,1),
('jingliyang','female',18,'20110211','teacher',9000,401,1),
('jinxin','male',18,'19000301','teacher',30000,401,1),
('成龙','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)
;

查询操作

select * from employee;
select distinct post from employee;
select name,salary*12 from employee;
select name,salary*12 as annual_salary from employee;
select concat('姓名:',name,'年薪:',salary*12) as annual_salary from emplyee;

where约束

        1.比较符:>< >= <= <> !=

         2.between 80 and 100 值在80到100之间

         3.in(80,90,100) 值是80或90或100

         4.like 'egon%'

               pattern可以是%或_,

               %表示任意多个字符

               _表示一个字符

          5.逻辑运算符:在多个条件直接

#单表查询
select name from employee where post='sale';
#多表查询
select name,salary from employee where post='teacher' and salar>10000;
#关键字查询
select name,salary from employee where salary between 10000 and 20000;
#关键字 is null 判断null只能用is
select name,post_comment from employee where post_comment is null;
#关键字in
select name,salary drom employee where salary in (3000,3500,4000,9000);
#关键字like模糊查询
select * from employee where name like 'eg%';
select * from employee where name like 'al__';

分组查询group by

查看mysql默认下的sql_mode:
select @@global.sql_mode;  
设置sql_mode
set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

select * from employee group by post;
select post,group_concat(name) from employee group by post;
select post,count(id) as count from employee group by post;

聚合函数

select count(*) from employee;
select count(*) from employee where depart_id = 1;
select max(salary) from employee;
select min(salary) from employee;
select avg(salary) from employee;
select sum(salary) from employee;

having过滤

select post,avg(salary) as new_sa from employee where age>=30 group by post having avg(salary) >10000;

distinct

select count(distinct post) from employee;

查询排序:order by

select * from employee order by salary;
select * from employee order by salary asc;#升序
select * from employee order by salary desc; #降序
select * from employee order by age;
select * from employee order by age,salary desc;

限制查询条件:limit

select * from employee order by salary desc limit 3;
select * from employee order by salary desc limit 0,5; #从0开始显示5条
select * from employee order by salary desc limit 5,5; #从5开始显示5条

正则表达式查询

select * from employee where name regexp '^alex';
select * from employee where name regexp 'on$';
select * from employee where name regexp 'm{2}';