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MySQL:数据库入门篇2

发布时间:2020-12-11 23:56:47 所属栏目:MySql教程 来源:网络整理
导读:#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

#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),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)

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