数据库2

用户权限

1，创建用户

create user 'ergou'@'127.0.0.1' identified by '123';

2移除用户

drop user 'ergou'@'127.0.0.1';

3修改用户

rename user 'ergou'@'127.0.0.1' to 'er'@'127.0.0.1' 

4 查看授权

show grants for 'er'@'127.0.0.1';

5，授权

手动逐个授权

GRANT SELECT ,UPDATE on a1.info1 to  'er'@'127.0.0.1';

授权所有

grant all privileges on *.* to 'er'@'127.0.0.1';

6 移除授权

revoke all PRIVILEGES on *.* from 'er'@'127.0.0.1';

7开放外部访问权限

CREATE user 'ergou'@'%' IDENTIFIED by '123';
GRANT all PRIVILEGES on *.* to 'ergou'@'%';
FLUSH PRIVILEGES;#刷新权限

修改密码

第一种方式

mysqladmin -u用户名 -p原密码 password 新密码;

mysqladmin -uergou -p123 password 111

第二种方式

set password for 'ergou'@'%'=password('44');

第三种方式

update mysql.user set password = password('123') where user ='ergou' and host ='%';
flush PRIVILEGES;

忘记密码

1.关闭mysql服务
2.重新启动mysql服务并跳过权限表
3.直接通过mysql登录
4.修改密码
5.刷新

单表查询

1聚合函数

SELECT SUM(name),AVG(age),max(age),min(age),count(name) FROM info1;

2分组

select sum(salary),dept_id from person GROUP BY dept_id
        
        select sum(salary) as w ,dept_id from person GROUP BY dept_id HAVING w >20000
        
        -- 查询每个部门的平均薪资 并且看看这个部门的员工都有谁?
        select avg(salary),dept_id,GROUP_CONCAT(name)  from person GROUP BY dept_id

        
        #查询平均薪资大于10000的部门, 并且看看这个部门的员工都有谁? 

        select avg(salary),dept_id,GROUP_CONCAT(name)  from person GROUP BY dept_id HAVING 
        avg(salary) >10000

3，分页

select * from person LIMIT 8,4
        ps: limit (起始条数),(查询多少条数);

4.SQL 语句关键字的执行顺序

执行顺序: FROM -> WHERE -> GROUP BY -> HAVING -> SELECT -> ORDER BY ->limit　

5 多表联合查询

select * from person p,dept d where p.dept_id = d.did  -- 笛卡尔乘积
        
     
        -- 多表联合查询
        -- select * from person p,dept d where p.dept_id = d.did  -- 笛卡尔乘积
        -- 
        -- 
        -- -- 左连接查询
        -- select * from person LEFT JOIN dept on person.dept_id = dept.did;
        -- 
        -- -- 右连接查询
        -- select * from person RIGHT JOIN dept on person.dept_id = dept.did;
        -- 
        -- -- 内连接查询
        -- select * from person INNER JOIN dept on person.dept_id = dept.did;


        -- 全连接
        select * from person LEFT JOIN dept on person.dept_id = dept.did
        UNION 
        select * from person RIGHT JOIN dept on person.dept_id = dept.did;


        select * from person LEFT JOIN dept on person.dept_id = dept.did
        UNION all
        select * from person RIGHT JOIN dept on person.dept_id = dept.did;
        
        
    6. 复杂条件查询
    -- 1. 查询出 教学部 年龄大于20岁,并且工资小于4000的员工,按工资倒序排列.
        -- (要求:分别使用多表联合查询和内连接查询)

        select did from dept where dname ='教学部';

        select * from person where age>20 and 
        dept_id =(select did from dept where dname ='教学部') and salary <10000 ORDER by salary DESC
        
    -- 2.查询每个部门中最高工资和最低工资是多少,显示部门名称

        select MAX(salary),min(salary),dname from person 
    LEFT JOIN dept ON person.dept_id = dept.did GROUP BY dept_id    
        
        
    7.子语句查询
        1.使用结果集作为表名查询
            select * from (SELECT * from person) as aaa
        
        --  2.求最大工资那个人的姓名和薪水

        select max(salary) from person;

        select* from person where salary = (select max(salary) from person);
                
    --  3. 求工资高于所有人员平均工资的人员

        select avg(salary) from person;

        select * from person where salary >(select avg(salary) from person)
        

 临时表查询

SELECT * FROM person as p1 ,
(SELECT  dept_id,avg(salary) AS m FROM person GROUP BY dept_id) as p2 
where p1.dept_id=p2.dept_id and p1.salary>p2.m;

将得出的结果作为新的表进行联合查询

判断查询

SELECT p.name,p.age,p.sex,p.salary, if(p.salary>10000,'高端人群','低端人群')as '级别' FROM person p;

SELECT  dname  , SUM(case when salary>10000 then 1 else 0  END) as '富人',
                             SUM(case when salary BETWEEN 5000 and 10000 then 1 else 0 end ) as '小资',
                             SUM(case when salary BETWEEN 3000 and 5000 then 1 else 0 end ) as '平民',
                             SUM(case when salary BETWEEN 1000 and 3000 then 1 else 0 end ) as '屌丝'    FROM person,dept WHERE dept.did=person.dept_id GROUP BY dept_id;

SQL逻辑查询语句执行顺序(重点***)

SELECT DISTINCT <select_list>

FROM <left_table>

<join_type> JOIN <right_table>

ON <join_condition>

WHERE <where_condition>

GROUP BY <group_by_list>

HAVING <having_condition>

ORDER BY <order_by_condition>

LIMIT <limit_number>

外键约束

约束是一种限制，它通过对表的行或列的数据做出限制来确保表的数据的完整性，唯一性

外键约束一般是在创建表的同时创建的，