106 Record operation

# 增：
insert into user select user,password from mysql.user;

# 删除：
delete from 表 where 条件

#改：
update 表 set 字段=值 where 条件；

#查：
#单表查询：
select distinct 字段1，字段2，字段3，...from 表名
                            where 过滤条件
                            group by 分组的条件
                            having 筛选条件
                            order by 排序字段
                            limit n；

简单查询：
select name,sex from emp;
select * from emp;

避免重复 distinct：
select distinct sex,post from emp;

进行四则运算：
select name as 名字,salary*12 as 年薪 from emp;

concat()拼接记录的内容
select name,concat(salary*12,"$") as 年薪 from emp;
select name,concat_ws(":","annual_salary",salary*12,"$") as 年薪 from emp;

select (
    case
    when name="egon" then
        name
    when name="egon" then
        concat(name,"_sb")


=================where===============
select * from emp where id >= 3 and id <=5;
select * from emp where id between 3 and 5;

select * from emp where id = 3 or id=5 or id=7;
select * from emp where id in (3,5,7);

select * from emp where id like "jin%"
select * from emp where id like "jin__"

select * from emp where name regexp "n$"; --正则

===============group by==============
select * from emp group by depart_id
分完组之后只能看到分组的字段以及聚合的结果
max()
min()
avg()
sum()
count()

select depart_id,count(id),avg(salary),max(age),min(salary),sum(salary) from emp group by depart_id;
# 每个部门都有多少个人
select depart_id,count(id) from emp group by depart_id;

# 每个职位男生的平均薪资
select post,avg(salary) from emp where sex="male" group by post;


select post, group_concat(name) from emp group by post;
select post, group_concat(name) from emp where sex="male" group by post;


===============having==============='
# having与where本质区别就是在于having是在分组之后发生过滤，可以使用聚合函数，where则不能。
select max(salary) from emp where max(salary) > 100000;

# 找出来男生平均薪资大于3000的职位
select post,avg(salary) from emp where sex="male" group by post having avg(salary) > 8000;

=================order by================
select * from emp order by salary;
 select * from emp order by salary desc;
 select * from emp order by age,id desc;

====================limit===================
# 可用于分页
select * from emp limit 0,5;  --0是起始位置，5是显示的记录条数
select * from emp limit 5,5;
select * from emp limit 10,5;
select * from emp limit 15,5;
select * from emp limit 20,5;