mysql笔记(二)--基本操作
文章目录
(一) mysql笔记–基本概念
(二) mysql笔记–基本操作
(三) mysql笔记–事务
(四) mysql笔记–索引
(五) mysql笔记–其他操作
2.1 数据库与表
2.1.1 基本操作
- 创建 create database db default charset utf8; 删除 drop database db;
- 查看 数据库 show databases; 表 show tables;
- 进入数据库 use databasesName;
- 查看表描述 describe myuser; 或show columns from myuser;
- 建表
CREATE TABLE myuser( id VARCHAR(10),name VARCHAR(10),sex CHAR(1),src VARCHAR(20) );
-- 指定引擎设置编码(innodb支持事务,myisam不支持)
create table t1(id int, name char(10)) engine=innodb charset=utf8;
- 清表与删表
delete from t1; -- 清空表内容,但重新插入自增键仍从上一次开始
truncate from t1; -- 速度更快,自增约束插入重1开始
drop table t1; -- 删除表
- 数值类型
一共有三种,数字型,时间类型,字符串型
- 日期按照格式选择,数值与字符串按照大小,超出则会Out of range value报错,字符串类型定长往前放,不定长往后放提高速度,长度大于255使用text存
- 金钱最好用int/bigint(整数,单位用分,拿出来进行*100换成元)/decimal(类似字符串保存),千万不要直接用浮点,会有精度损失.
- 文件类型一般存在服务器的文件系统,数据库保存其路径,还有枚举类型,集合等等
- 用户与授权
create user ‘用户名@’ip.%’ identified by ‘name’
drop ‘用户名@’ip.%’
grant 权限 on 数据库.表 to ‘用户’@’ip地址’
remove *.* from -- 使用通配符
2.1.2 更改属性
- 添加 ADD
AlTER TABLE user2 ADD PRIMARY KEY(id,name);
ALTER TABLE user5 ADD UNIQUE(name);
- 删除 DROP
AlTER TABLE user2 DROP PRIMARY KEY;
- 修改 MODIFY
AlTER TABLE user3 MODIFY id INT PRIMARY KEY;
2.2 插入
- 完整插入与半插
INSERT INTO myuser VALUES('01','ls','1','www.rice.cn');
INSERT INTO myuser(id,name) VALUES(02,’te’), (03,’te2’);
INSERT INTO myuser(id,name) SELECT * FROM myuser2;
2.3 删除
DELETE FROM myuser WHERE sex = '0';
DELETE FROM myuser; //全删
2.4 修改
UPDATE myuser SET sex = '1' , age =’18’ WHERE name = 'ls';
2.5 查询与函数
- 非重复
select distinct t_depart from teacher;
- 区间 between
select * from score where sc_degree between 60 and 80; -- 大于等于小于等于
select s_no,sc_degree,grade from score,grade
where sc_degree between low and upp;
- 集合in与union
select * from score where sc_degree in(85,86,88);
select * from score where s_no in
( select s_no from student where s_class = "95031");
SELECT s_name AS name, s_sex AS sex, s_birthday AS birthday FROM student
UNION
SELECT t_name AS name, t_sex AS sex, t_birthday AS birthday FROM teacher;
- 日期函数 now
select year(s_birthday),month(s_birthday),day(s_birthday) from student;
select s_name, year(now()) - year(s_birthday) age from student; //年龄计算
- 排序 asc
select * from score order by sc_degree asc; //默认升序
select * from score order by sc_degree desc; //降序
//以班级升序,相同则以成绩降序
select * from score order by c_no asc, sc_degree desc;
返回最大值 max
select max(sc_degree) from score;
select max(s_no) max,min(s_no) min from student;
选择区间条目 limit
select * from student limit 0 , 2; //选择 起始行,取的行数
- 平均值 avg
select avg(sc_degree) from score;
- 任意值any与全部all
- 查询3-105课号中大于3-245课号中成绩最低的
select * from score where c_no = '3-105' AND
sc_degree > any(select sc_degree from score
where c_no = '3-245' )
- 查询3-15课号中都大于3-245课号中成绩最高的
select * from score
where sc_degree > all (select sc_degree from score
where c_no = '3-245')
and c_no = '3-105';
- 分组 group
select c_no,avg(sc_degree) from score group by c_no; //每个班的平均分
- 模糊查询 like
select * from score where c_no like '3%'; //多个字符
select * from score where c_no like '3_'; //单个字符
- 过滤 having
-- where行级过滤,having组级过滤,与分组搭配使用
select c_no,avg(sc_degree) from score group by c_no having count(c_no) > 2;
- 多表查询与嵌套查询
- 连表
select s_name,sc_degree from student,score
where student.s_no = score.s_no;
select s_name,c_name,sc_degree from course,score ,student
where course.c_no = score.c_no
and student.s_no = score.s_no;
- 找出95031班的成绩
select * from score
where s_no in ( select s_no from student where s_class = "95031");
- 各班平均分
select c_no,avg(sc_degree) from score
where s_no in( select s_no from student where s_class = "95031")
group by c_no;
- 子查询
- 查询选修3-105大于109号学生成绩的
select * from score
where c_no = '3-105'
and sc_degree > (select sc_degree from score
where s_no = '109' and c_no = '3-105');
- 查询’张旭’老师任课的学生成绩
select * from score
where c_no in (
select c_no from course
where t_no = (select t_no from teacher where t_name = '张旭'));
- 查询选课人数大于5的教师名
select * from teacher
where t_no in(select t_no from course
where c_no in (
select c_no from score group by c_no having count(s_no) > 5));
- 求低于该科平均分的
select * from score a
where sc_degree < (select avg(sc_degree) from score b where a.c_no = b.c_no);
2.6 连接
2.6.1 内连接
inner join 或 join , 即两张表中的数据通过某个字段查询出相关记录
select * from person inner join card on person.cardId = card.id;
2.6.2 外连接
-
左连接 left join 或 left outer join,无相等则补NULL
-
右连接 right join 或 right outer join,无相等则补NULL
-
完全外连接 full join 或 full join outer join
mysql不支持full join 但可以使用左右连接然后使用union即可
