Mysql学习日记-02外键 ，索引， sql语句的补充
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主键：
一个表只能有一个主键
主键可以由多列组成
外键：
就是把一个表的成员 和 另外一个表的成员联系起来
比如 student 里有（学号，姓名） 而 score里有（学号，成绩） 这个时候就可以把 student（学号） = score （学号）关联起来
语法规则是 ：
create table score (
sid int not null auto_increment primary key,
number iint
)
create table student (
nid int not null auto_increment primary key,
name varchar(20),
constraint fk_xxx foreign key (nid ) references score (nid)
)
对于自增补充：
是对表里面的序号的步长设置
desc t10;
show create table t10;
show create table t10 G;
alter table t10 AUTO_INCREMENT=20;
mysql: 自增步长
基于会话级别：
show session variables like ‘auto_inc%‘; 查看全局变量
set session auto_increment_increment=2; 设置会话步长
# set session auto_increment_offset=10;
基于全局级别：
show global variables like ‘auto_inc%‘; 查看全局变量
set global auto_increment_increment=2; 设置会话步长
# set global auto_increment_offset=10;
SqlServer：自增步长：
基础表级别：
CREATE TABLE `t5` (
“nid` int(11) NOT NULL AUTO_INCREMENT,
`pid` int(11) NOT NULL,
`num` int(11) DEFAULT NULL,
PRIMARY KEY (`nid`,`pid`)
) ENGINE=InnoDB AUTO_INCREMENT=4, 步长=2 DEFAULT CHARSET=utf8
CREATE TABLE `t6` (
`nid` int(11) NOT NULL AUTO_INCREMENT,
`pid` int(11) NOT NULL,
`num` int(11) DEFAULT NULL,
PRIMARY KEY (`nid`,`pid`)
) ENGINE=InnoDB AUTO_INCREMENT=4, 步长=20 DEFAULT CHARSET=utf8
索引：
create table t1(
id int ....,
num int,
xx int,
unique 唯一索引名称 (列名,列名),
constraint ....
)
索引即是绑定一对一 加速查找
约束constraint不能重复（可以为null） 主键不能重复（不能为null）
外键与索引的结合 ：
比如 一个用户只能拥有一个博客 ，一对一
create table userinfo1(
id int auto_increment primary key,
name char(10),
gender char(10),
email varchar(64)
)engine=innodb default charset=utf8;
create table admin(
id int not null auto_increment primary key,
username varchar(64) not null,
password VARCHAR(64) not null,
user_id int not null,
unique uq_u1 (user_id),
CONSTRAINT fk_admin_u1 FOREIGN key (user_id) REFERENCES userinfo1(id)
)engine=innodb default charset=utf8;
多对多 ：
用户表
主机表
用户主机关系表
create table userinfo2(
id int auto_increment primary key,
name char(10),
gender char(10),
email varchar(64)
)engine=innodb default charset=utf8;
create table host(
id int auto_increment primary key,
hostname char(64)
)engine=innodb default charset=utf8;
create table user2host(
id int auto_increment primary key,
userid int not null,
hostid int not null,
unique uq_user_host (userid,hostid),
CONSTRAINT fk_u2h_user FOREIGN key (userid) REFERENCES userinfo2(id),
CONSTRAINT fk_u2h_host FOREIGN key (hostid) REFERENCES host(id)
)engine=innodb default charset=utf8;
总结：对sql语句数据行的操作补充 必须注重细节！括号 逗号不要忘
逻辑关系要理顺
create table tb12(
id int auto_increment primary key,
name varchar(32),
age int
)
增 ：
inset into tb12(name ,age ) values(‘kk‘，12)；
inset into tb12 (name,age) values(‘kk‘,12),(‘alex‘,13);
insert into tb12(name,age) select name,age from tb11;
删：
delete from tb12;
delete from tb12 where id !=2
delete from tb12 where id =2
delete from tb12 where id > 2
delete from tb12 where id >=2
delete from tb12 where id >=2 or name=‘alex‘
改：
update tb12 set name=‘alex‘ where id>12 and name=‘xx‘
update tb12 set name=‘alex‘,age=19 where id>12 and name=‘xx‘
查：
select * from tb12;
select id,name from tb12;
select id,name from tb12 where id > 10 or name =‘xxx‘;
select id,name as cname from tb12 where id > 10 or name =‘xxx‘;
select name,age,11 from tb12;
其他：
select * from tb12 where id != 1
select * from tb12 where id in (1,5,12);
select * from tb12 where id not in (1,5,12);
select * from tb12 where id in (select id from tb11)
select * from tb12 where id between 5 and 12;
通配符：
select * from tb12 where name like "a%"
select * from tb12 where name like "a_"
分页：
select * from tb12 limit 10;（10页）
select * from tb12 limit 0,10;（从第一个开始数，10个）
select * from tb12 limit 10,10;
select * from tb12 limit 20,10;
select * from tb12 limit 10 offset 20;
排序：
select * from tb12 order by id desc; 大到小 记忆 D大
select * from tb12 order by id asc; 小到大 记忆 a小
select * from tb12 order by age desc,id desc;
分组：用于数目的统计 比如表中一个学生可以选择多个课程 用group by students 统计下来其实只有一个人
select count(id),max(id),part_id from userinfo5 group by part_id;
count
max
min
sum
avg
**** 如果对于聚合函数结果进行二次筛选时？必须使用having ****
select count(id),part_id from userinfo5 group by part_id having count(id) > 1;
select count(id),part_id from userinfo5 where id > 0 group by part_id having count(id) > 1;
连表操作：
其实可要记住一个就行了
select * from userinfo5,department5
select * from userinfo5,department5 where userinfo5.part_id = department5.id
select * from userinfo5 left join department5 on userinfo5.part_id = department5.id
select * from department5 left join userinfo5 on userinfo5.part_id = department5.id
# userinfo5左边全部显示
# select * from userinfo5 right join department5 on userinfo5.part_id = department5.id
# department5右边全部显示
select * from userinfo5 innder join department5 on userinfo5.part_id = department5.id
将出现null时一行隐藏
select * from
department5
left join userinfo5 on userinfo5.part_id = department5.id
left join userinfo6 on userinfo5.part_id = department5.id
select
score.sid,
student.sid
from
score
left join student on score.student_id = student.sid
left join course on score.course_id = course.cid
left join class on student.class_id = class.cid
left join teacher on course.teacher_id=teacher.tid
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