#移除主键时需要先解除递增，才能解除主键
alter table info modify id int null , drop PRIMARY key
一.用户权限
1.创建用户
create user ‘hanshe‘@‘127.0.0.1‘ IDENTIFIED by ‘123‘; -- 创建用户
2.移除用户
drop user ‘hanshe‘@‘127.0.0.1‘ ; -- 移除用户
3.修改用户
RENAME user ‘hanshe‘@‘127.0.0.1‘ to ‘hanxiaoqiang‘@‘192.168.0.1‘ -- 修改用户
4.查看授权
show GRANTS for ‘hanshe‘@‘127.0.0.1‘;-- 查看用户 权限
5.授权
GRANT select,update ON db1.info to ‘hanshe‘@‘127.0.0.1‘;-- 授权
GRANT all PRIVILEGES on *.* to ‘hanshe‘@‘127.0.0.1‘; -- 授权所有权限
6.移除授权
REVOKE all PRIVILEGES on *.* FROM ‘hanshe‘@‘127.0.0.1‘; -- 移除权限
7.开放外部访问权限
create user ‘test‘@‘%‘ identified by ‘123‘;
GRANT all PRIVILEGES on *.* to ‘test‘@‘%‘;
FLUSH PRIVILEGES; -- 刷新权限
二.修改用户密码
1.方式一:使用 mysqladmin 命令
mysqladmin -u用户名 -p原密码 password 新密码;
2.方式二:直接设置密码
set password for ‘hanshe‘@‘%‘ = password(‘166‘)
3.方式三: 直接修改
update mysql.user set password = password(‘123‘) where user =‘hanshe‘ and host =‘%‘
flush PRIVILEGES;
5.7 版本
update mysql.user set authentication_string = password(‘123‘) where user =‘hanshe‘ and host =‘%‘;
flush PRIVILEGES;
三.忘记密码怎么办（本地使用数据库）
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
四单表查询
1.聚合函数
select sum(name),avg(age),max(age),min(age),count(name) FROM person;
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
五. 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;
六、 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
七.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)