关系数据库的注释
-- 单行注释
/* 多行注释 */
数据类型
数值
整数 只能保存整数,如果是小数，则自动四舍五入取整。
mediumint
smallint
tinyint(6) zerofill
int(8)
bigint
decimal(8,0)
小数实型
float(p,s)
numeric(p,s)
decimal(p,s) decimal
double(p,s)
字符串
char() 固定长度字符串 手机号 学号 身份证号
sno char(12)
insert into 1 6
varchar() 可变长度字符串 家庭地址
varchar(12)
text
longtext
日期 时间类型
日期 date 2020-06-28
时间 time 16:48:50
date
time
datetime
timestamp
日期时间 datetime 2020-06-28 16:48:50
复合数据类型
enum 枚举 enum(‘计算机科学‘,‘英语‘,‘会计‘)
enum(‘Y‘,‘N‘)
set 集合
set(‘java‘,‘php‘,‘python‘,‘javascript‘,‘mysql‘)
‘java‘ true
‘php,java,mysql‘ true
‘abc,java,ok‘ false
mysql 5.7新的json 类型 NoSQL类型
二进制数据类型
如果将一张图片文件，保存到数据库，必须使用二进制类型。
longblob
表操作 建立表
create table if not exists 表名(
字段名 数据类型 unsigned zerofill auto_increment 相关的约束,
)引擎 字符集 自增;
删除表
drop if exists table 表名,表2,表3;
表的约事，字段类型的修饰
unsigned zerofill auto_increment 自增一般和主键
not null 不能为空
默认值
default 18
default ‘郑州‘
default ‘男‘
主要约束
PK FK UK 默认值 default 非null not null
PK primary key 主键 唯一的，必须填写，不能为null 空 一表只能有一个主键
FK foreign key 外键一般两个表以表
uk unique 唯一约束
sno char(5) primary key
id int unsigned auto_increment,
primary key(id)
primary key(id,name) 复合主键，一个主键控制两列或多列
foreign key 外键操作
select host,user,plugin from mysql.user;
-- 建立老师信息表
create table teacher(
tid int unsigned auto_increment,
tname varchar(15) not null,
primary key(tid)
)engine=innodb charset=utf8 auto_increment=1;
-- 插入数据
insert into teacher(tname) values(‘李老师‘),(‘张老师‘),(‘周老师‘);
insert teacher(tname) values(‘李老师‘);
select * from teacher;
/* 建立学生信息表 */
create table student(
sid int unsigned auto_increment,
sname varchar(15) not null,
stid int unsigned,
primary key(sid)
/*foreign key(stid) references teacher(tid)*/
/* constraint fk foreign key(stid) references teacher(tid) on delete cascade*/
)engine=innodb charset=utf8 auto_increment=1;
-- 给student表增加建立外键
alter table student add foreign key(stid) references teacher(tid);
-- alter table student add foreign key(stid) references teacher(tid) on delete set null on update cascade;
drop table student;
select * from teacher;
select * from student;
insert into student values(null,‘李四‘,null),(null,‘jack‘,2),(null,‘张三‘,5),(null,‘李强‘,5);
select * from student;
select * from teacher;
delete from teacher where tid = 3;
select 语句查询
select * from student;
select * from xxx;
select 条件
= > < >= <= != <>
-- 随机查询两条记录 (此语句对于大型数据不合理)
select name,score,age from stu order by rand() limit 2;
--
select * from mysql.user;
select 10*10,10/5,10+5;
-- = > >= < <= != <>
select * from stu where name = ‘李四‘;
select * from stu where age != 20;
update stu set address = null where id = 5;
-- is null is not null
select * from stu where address is null;
select * from stu where address is not null;
-- in not in
select * from stu where dept in (‘英语‘);
select id,name from stu where id not in (1,2,4,6);
-- between and not between and
select id,name,age from stu where age not between 18 and 20;
select id,name,age from stu where age <18 or age >20;
select id,name,age from stu where age between 18 and 20;
select id,name,age from stu where age >=18 and age <=20;
-- and &&
-- or ||
-- not 相当于java ! 非
select * from stu where sex = ‘女‘;
-- 模糊查询 like % 代表0个或多个任意字符 _ 一个任意字符
select * from stu where name like ‘张%‘;
select * from stu where name like ‘_三_‘;
select * from stu where name like ‘%丰%‘;
select * from stu where name like ‘李%‘;
-- 正则表达式条件
select * from stu where name regexp ‘^..$‘;
查询排序 order asc desc;
select * from stu;
查询聚合 聚集 集合函数
-- 查询班级的总人数
select count(*) from stu;
-- 查询多少个专业
-- 查询统计专业的个数 count(
select count(distinct dept) from stu;
select distinct * from stu;
-- 查询分组统计 group by order by where limit
group by 要集合函数配置
select max(score) from stu where sex = ‘男‘;
select max(score) from stu where sex = ‘女‘;
UML
E-R