mysql数据库、表、字段、记录：增、删、改、查

数据库、表、字段、记录：增、删、改、查。分类进行学习，重点是查询。

/*
数据库、表、字段、记录：增、删、改、查

*/

-- 1.数据库：增删改查
create datebase if not exists jkxy;
drop database if exists jkxy;
rename database jkxy to jikexueyuan; --这条语句官方说不能用了
show databases;
use jikexueyuan;
/*
    1、CREATE DATABASE new_db_name;  创建新的数据库
    2、RENAME TABLE db_name.table1 TO new_db_name.table1;  对所有的表进行重命名
    3、DROP DATABASE db_name;            删除原来的数据库
*/

-- 2.表：增删改查
create table if not exists user();
drop table if exists user;
alter table user rename users; --as可以省略不要
show tables;
desc users;

--3.字段：增删改查
alter table user add tpass char(32) not null default ‘‘ after userpass;
alter table user drop tpass;
alter table change userpass tpass varchar(50) not null default ‘‘;
alter table modify tpass char(32) not null default ‘‘;
desc user;
alter table user auto_increment = 1; --设置自增值为1
alter table user add index username_index(username); --为字段添加索引
alter table user drop index username_index;
alter table user add unique(tel); --还剩主键不知道怎么添加和删除
alter table user drop index tel;  --alter table user add unique tel_unique(tel); index和unique删除都是用drop index
show indexes from user;

--4.记录：增删改查
insert into user(username, pwd, email) values(‘user1‘, md5(123456), ‘[email protected]‘), (‘user2‘, md(123456), ‘[email protected]‘);
delete from user where id = 2;
update user set username = ‘user111‘, email = ‘[email protected]‘ where id = 1;
select username from user where id > 1;

--5.重点：查询
select username from user where id > 2;
select username as ‘用户名‘ from user where id > 2;
select distinct email from user; --只针对一个字段
select * from user where username like ‘%user%‘; --内容包含user的记录

select * from user order by score DESC;
select * from user order by score DESC limit 2; --limit 0, 2;
select * from user order by score DESC limit 2,3; --下标从0开始，从第2+1条开始，取三条

select count(id) num from user where id > 2; --计算id的条数，num是一个别名
select avg(score) num from user; --计算平均值，还有函数：max() min() sum()