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；
执行顺序：
执行顺序并不是按照语法来执行的，
1.先找到表：from
2.拿到where指定的约束条件，去文件/表中取出一条条记录
3.将取出的一条条记录进行分组group by ，如果没有group by ，则整体作为一组
4.将分组的结果进行having 过滤
5.执行select
6.去重 ：distinct
7.将结果按条件排序：order by
8.限制结果的显示条数 ：limit
company.emp 员工id id int 姓名 emp_name varchar 性别 sex enum 年龄 age int 入职日期 hire_date date 岗位 post varchar 职位描述 post_comment varchar 薪水 salary double 办公室 office int 部门编号 depart_id int #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 ); #查看表结构 mysql> desc emp; +--------------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | NO | | NULL | | | sex | enum('male','female') | NO | | male | | | age | int(3) unsigned | NO | | 28 | | | hire_date | date | NO | | NULL | | | post | varchar(50) | YES | | NULL | | | post_comment | varchar(100) | YES | | NULL | | | salary | double(15,2) | YES | | NULL | | | office | int(11) | YES | | NULL | | | depart_id | int(11) | YES | | NULL | | +--------------+-----------------------+------+-----+---------+----------------+ #插入记录 #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 ('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) ; #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
二 、where 约束
1、where字句中可以使用
1.比较运算符：>，<，>=，<=，!=
2.between 10 and 20 值在10到20之间
3.in (10,20,30) 值是10或20或30
4.like'egon%'
%表示任意多字符
_表示任意一个字符
5.逻辑运算符：在多个条件直接可以使用逻辑运算符and or not
1、select id,name from db8.emp where id >=3 and id <=6 select * from db8.emp where id between 3 and 6; 2、select * from db8.emp where salary =20000 or salary =18000 or salary = 17000; select * from db8.emp where salary in (20000,18000,17000); 3、要求：查询员工姓名中包含i字母的员工姓名与薪资 select name,salary from db8.emp where name like '%i%'; 4、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 db8.emp where name like '____'; select name,salary from db8.emp where char_length(name) = 4; 5、select * from db8.emp where id not between 3 and 6; select * from db8.emp where salary not in (20000,18000,17000); 6、 要求：查询岗位描述为空的员工名与岗位名 select name,post from db8.emp where post_comment is NULL; select name,post from db8.emp where post_comment is not NULL;
三、group by 分组
1、为什么要分组
1.分组是发生在where之后，即分组是基于where之后得到的记录而进行的
2.分组指的是：将所有的记录按照某个相同字段进行归类，比如针对员工信息表的职位分组，或者按照性别进行分组等
3.为何要分组呢？
取每个部门的最高工资
取每个部门的员工数
取男人数和女人数
小窍门：‘每’这个字后面的字段，就是我们分组的依据
4、大前提：
可以按照任意字段分组，但是分组完毕后，比如group by post，只能查看post字段，如果想查看组内信息，需要聚合函数
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 globl sql_mode='strict_trans_tables,only_full_group_by'; 用聚合函数： 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）用于字符拼接 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; # 查询四则运算 select name,salary*12 as annual_salary from emp;
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;
distinct去重
select distinct post,avg(salary) from emp where age >= 30 group by post having avg(salary) > 10000;
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
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;
正则表达式
select * from emp where name regexp '^jin.*(n|g)$';
