关键字：
1、简单查询select,from
2、条件查询where,limit
3、排序查询order by desc | asc
4、分组查询group by, having
5、嵌套查询select from (select ...)
6、多表查询inner join, outer join,...
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1、简单查询
起别名：列别名，表别名
select first_name as name ,salary from employees as e;
select * from employees e;
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2、条件查询：
select first_name, salary hire_date from hr.employees where salary > 10000 limit 10;
limit num 限制数量，排好序
select first_name, salary hire_date 执行顺序：4
from hr.employees 执行顺序：1
where salary > 10000 执行顺序：2
limit 10; 执行顺序：3
where salary between 10000 and 20000;
where salary > 10000 and salary < 20000;
and 可以使用符号 “&&”替换
where salary = 10000 or salary = 20000;
where salary = 10000 || salary = 20000;
where salary in (10000,20000);
or 可以使用符号 “||”替换
select first_name, salary from hr.employees order by salary desc limit 5; 前5条数据
select first_name, salary from hr.employees order by salary desc limit 4, 10;第5到往后10条数据
select first_name, salary from hr.employees order by salary desc limit 2 offset 4;第4条后2条
limit 5 offset 0; 等于 limit 5 等于 limit 0,5;
limit子句通常用于分页查询
取当前页数据：
select * from tb_name
limit (当前页-1)*每页最多显示的据路数，每页最多显示的记录数
假如有limit子句，一定记得，放在所有语句的最后面。最后执行
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3、排序查询：
select first_name ,salary from hr.employees order by salary desc;
select first_name ,salary from hr.employees order by 2 desc; 其中select列表中的第2列
select first_name ,salary sal from hr.employees order by sal desc; 其中sal是定义的别名
按工资降序排序，若工资相同，按入职日期升序排序。
select first_name, salary, hire_date from hr.employees order by salary desc, hire_date asc;
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4、分组查询：group by
一半放在from子句后面，where后面
假如有having，则一定有group by
说明：子查询一般会应用在where子句，from子句，having子句的后面，
假如是from子句后，需要给这个子查询起一个别名
求每个部门的平均薪水，只显示平均薪水大于5000的
select department_id, avg(salary)
from hr.employees
group by department_id
having avg(salary) > 5000;
求每个部门的平均薪水，只显示平均薪水大于5000的，部门id为空的除外
select department_id, avg(salary)
from hr.employees
where department_id is not null
group by department_id
having avg(salary) > 5000;
select department_id ,count(*), max(salary), min(salary),sum(salary)
from hr.employees
where department_id is not null
group by department_id
order by count(*) desc;
练习:
查询每个工种job_id的平均薪水，只显示平均薪水大于5000的？
select job_id, avg(salary)
from employees
group by job_id
having avg(salary) > 5000;
求每年入职的人数，年份按照降序排序？
select year(hire_date),count(*) 每年入职总人数
from employees
group by year(hire_date)
order by year(hire_date) desc;
求每个部门薪水的最大值，降序排序，取前三名
select department_id, max(salary)
from employees
group by department_id
order by max(salary) desc
limit 3;
求每个部门中薪水最高的那个人的名字？
select department_id,first_name,max(salary)
from employees
group by department_id;
错误，这里的first_name 应该是组内第一个遇到的名字
方法一：不是所有数据库都支持
select department_id, first_name, salary from employees
where (department_id, salary) in (
select department_id,max(salary) from employees
where department_id is not null
group by department_id
);
求人数最多的那个部门的部门名称；
方法一：
select department_name from departments where department_id = (
select department_id from employees group by department_id order by count(*) desc limit 1
);
方法二：
select department_name
from departments
where department_id = (
select department_id
from employees
group by department_id
having count(*)=(
select max(ct)
from (
select count(*) ct
from employees
where department_id is not null
group by department_id)e));
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5、嵌套查询：
求比部门编号80平均薪水还要高的部门id以及平均薪水，部门id不为null;
select department_id, avg(salary) from employees
where department_id is not null
group by department_id
having avg(salary) > (
select avg(salary) from employees
where department_id = 80
group by department_id
);
group分组后的限制条件用having
有having，则必有group by。
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6、多表查询：
1）、多表查询的应用
当需要的数据来自于多张表时，考虑使用多表查询
select表内存在共有列时，应该在列名前加上表名进行标识区别。
求雇员编号201的员工所在部门的名称？(嵌套)
select department_name from departments where department_id = (
select department_id from employees where employee_id = 201
);
select first_name, e.department_id, department_name
from employees e, departments d
where e.department_id = d.department_id
and e.employee_id = 201;
select first_name, e.department_id, department_name
from employees e inner join departments d
on e.department_id = d.department_id
where e.employee_id = 201;
2）、左连接&右链接
求每个雇员及这个雇员所在的部门信息？
select employee_id, first_name, department_name
from employees e join departments d
on e.department_id = d.department_id;
错误：但是那个department_id是null的员工被遗漏了
select employee_id, first_name, department_name
from employees e left outer join departments d
on e.department_id = d.department_id;
其中”left outer“为左外链接，outer可以省略，
左外链接指的是左边表所有记录都显示，右边表只显示满足条件的。
求每个部门及各个部门中的雇员信息？
select department_name, employee_id, first_name
from employees e right outer join departments d
on e.department_id = d.department_id;
其中”left outer“为右外链接，outer可以省略，
右外链接指的是右边表所有记录都显示，左边表只显示满足条件的。
左外链接 与 右外链接 只要把表的左右相对位置调换其实是一样的。
3）、笛卡尔积（多表查询没写链接条件）
select department_name, employee_id, first_name
from employees e, departments d;
作用：通常借助笛卡尔积的结果作为测试数据用。快速建表
create table temp
as
select department_name, employee_id, first_name
from employees e, departments d;
数据表temp的记录和结构douban来自于as语句后的查询
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7、视图（view）
什么是视图？
视图是存储在数据库服务端的一张虚拟的表，视图只有结构，没有数据
所有的数据都是存储在表（table）中
视图的作用？
1、屏蔽外界直接对基表的访问，提高系统安全
2、简化sql语句在程序中的编写过程。
视图的创建？
create view myview1
as
select department_id, max(salary)
from employees
where department_id is not null
group by department_id;
一般不建议修改视图
视图的删除
drop view myview1;
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7、索引（Index）数据结构树的理解。
1）、索引是什么？书的大纲
2）、索引的作用 提高查询的效率
数据量比较大，而且经常性的对某些字段进行查询，此时建议建立索引
查询的时候，某些字段出现了大量的null值，建议建立索引
3）、索引的创建
create index index01 on employees (first_name);
索引创建好以后，会在查询时自动生效。（无须显式使用）
使索引失效
alter table tb_name disable key index01;
4）、索引的删除
drop index index01 on employees(first_name);
练习：
1 求薪水和最多的那个部门的部门名称？
嵌套查询：
select department_name from departments where department_id = (
select department_id from employees group by department_id order by sum(salary) desc limit 1
);
方案二：
select department_name
from departments
select department_id
where department_id = (
from employees
group by department_id
having sum(salary) = (
select max(sm)
from (
select sum(salary) sm
from employees
group by department_id
)e));
方案三：
select department_name
from departments d1 join (
select department_id, sum(salary) sm
from employees
group by department_id
having sum(salary) = (
select max(sm)
from (
select sum(salary) sm
from employees
group by department_id
)e))d2
on d1.department_id = d2.department_id;
多表查询：
select department_name
from employees e right join departments d
on e.department_id = d.department_id
group by d.department_id
order by sum(salary) desc
limit 1;
2 求每个部门中薪水最高的那个人的名字？
select first_name
from employees
where (department_id, salary) in (
select department_id, max(salary)
from employees
group by department_id
);
方案二：
select e1.department_id, first_name, salary
from employees e1 join (
select department_id, max(salary) max_sal
from employees
group by department_id
) e2
on e1.department_id = e2.department_id
where e1.salary = e2.max_sal;
3 求每个部门中薪水最高的前两名雇员的名字？
select department_id, first_name, salary
from employees e1
where department_id is not null
and 2 > (
select count(*)
from employees e2
where e1.department_id = e2.department_id
and e1.salary < e2.salary
);
本文出自 “一剑围城” 博客，请务必保留此出处http://weijiancheng.blog.51cto.com/10190955/1700535
原文地址：http://weijiancheng.blog.51cto.com/10190955/1700535
