SELECT 　　字段1 [as alias],字段2...　　FROM 表名
                  　　　　　　　　　　　　WHERE 条件
                  　　　　　　　　　　　　GROUP BY field
                  　　　　　　　　　　　　HAVING 筛选
                  　　　　　　　　　　　　ORDER BY field
                  　　　　　　　　　　　　LIMIT 限制条数

from
where
group by
having
select
distinct
order by
limit

1.找到表:from
2.拿着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
7.将结果按条件排序：order by
8.限制结果的显示条数

二 where过滤

    select id,name from db39.emp where id >= 3 and id <= 6
    select *  from db39.emp where id between 3 and 6;


    select * from emp where salary = 20000 or salary = 18000 or salary = 17000;
    select * from emp where salary in (20000,18000,17000);


    要求：查询员工姓名中包含i字母的员工姓名与其薪资
    select name,salary from db39.emp where name like '%i%'

    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 db39.emp where name like '____';
    select name,salary from db39.emp where char_length(name) = 4;



    select *  from db39.emp where id not between 3 and 6;
    select * from emp where salary not in (20000,18000,17000);

    要求：查询岗位描述为空的员工名与岗位名
    select name,post from db39.emp where post_comment is NULL;
    select name,post from db39.emp where post_comment is not NULL;


三 group by分组
    #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 global sql_mode="strict_trans_tables,only_full_group_by";

    #每个部门的最高工资
    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）
    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;

    # 补充concat（不分组时用）
    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
    mysql> select emp.id,emp.name from emp as t1; # 报错
    mysql> select t1.id,t1.name from emp as t1;


    # 查询四则运算
    select name,salary*12 as annual_salary from emp;


    分组练习

    1. 查询岗位名以及岗位包含的所有员工名字
        select post,group_concat(name) from emp group by post;


    2. 查询岗位名以及各岗位内包含的员工个数
        select post,count(id) from emp group by post;

    3. 查询公司内男员工和女员工的个数
        select sex,count(id) from emp group by sex;

    4. 查询岗位名以及各岗位的平均薪资
        select post,avg(salary) from emp group by post;
    5. 查询岗位名以及各岗位的最高薪资
    6. 查询岗位名以及各岗位的最低薪资
    7. 查询男员工与男员工的平均薪资，女员工与女员工的平均薪资
        select sex,avg(salary) from emp group by sex;

    8、统计各部门年龄在30岁以上的员工平均工资
       select post,avg(salary) from emp where age >= 30 group by post;


四 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;



五 distinct去重

select distinct post,avg(salary) from emp
            where age >= 30
            group by post
            having avg(salary) > 10000;

基础：两张表不加条件简单连表为求笛卡尔积
select * from emp,dep 等价于---》
select * from emp join dep
1、内连接：把两张表有对应关系的记录连接成一张虚拟表
select * from emp inner join dep on emp.dep_id = dep.id;
# 不加inner应该默认就是inner
#应用：
    select * from emp,dep where emp.dep_id = dep.id and dep.name = "技术"; # 不要用where做连表的活

    select * from emp inner join dep on emp.dep_id = dep.id
        where dep.name = "技术"
    ;

2、左连接：在内连接的基础上，保留左边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id;


3、右连接：在内连接的基础上，保留右边没有对应关系的记录
select * from emp right join dep on emp.dep_id = dep.id;


4、全连接：在内连接的基础上，保留左、右边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id
union
select * from emp right join dep on emp.dep_id = dep.id;



#补充：多表连接可以不断地与虚拟表连接

查找各部门最高工资
select t1.* from emp as t1
inner join
(select post,max(salary) as ms from emp group by post) as t2
on t1.post = t2.post
where t1.salary = t2.ms
;