自增补充
这是查看怎么创建的表， G示旋转90度显示表的内容
表的自增的关键是** AUTO_INCREMENT=3**，在表中添加数据后，这个会自动改变，通过alert可以改变这个默认值
mysql> show create table t1 G;
*************************** 1. row ***************************
Table: t1
Create Table: CREATE TABLE `t1` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` char(10) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8
下一次添加的内容的id会从20处添加
alter table t10 AUTO_INCREMENT=20;
自增步长
mysql是的默认步长是基于会话session的，sqlserver是基于表的。
查看全局变量，其中默认是1
mysql> show session variables like 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
+--------------------------+-------+
2 rows in set (0.00 sec)
设置步长基于会话步长，只能自该自己的会话
set session auto_increment_increment=2; 设置会话步长
set session auto_increment_offset=10; 设置开始的位置
基于全局的级别的，可以修改全部的会话
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;
唯一索引
unique
create table t1(
id int ....,
num int,
xx int,
unique 唯一索引名称 (列名,列名),
constraint ....
)
这了唯一的意思是：
- 约束不能重复（可以为空）
- 主键不能重复（不能为空）
作用是加速查找
外键的变种
-
单列
-
联合
关联一对多
create table userinfo(
id int auto_increment primary key,
username varchar,
usertype int,
)engine=innodb default charset=utf8;
create table admin(
id int auto_increment primary key,
user_id int,
passwprd varchar,
unique index(user_id),
constraint fk_key1 foreign key (user_id) references userinfo(id)
)engine=innodb default charset=utf8;
-- 外键关联多个列
create table t2(
nid int not null auto_increment,
pid int not null,
num int,
primary key(nid,pid)-- 这里的关联两个列的主键
)engine=innodb default charset=utf8;
create table t3(
id int auto_increment primary key,
name char,
id1 int,
id2 int,
constraint fk_t3_t2 foreign key (id1,id2) references t2(nid,pid)
)engine=innodb default charset=utf8;
多对多
-- 多对多
-- 用户表
create table userinfo(
id int auto_increment primary key,
username varchar,
gender int,
)engine=innodb default charset=utf8;
-- 主机表
create table computer(
id int auto_increment primary key,
name varchar,
)engine=innodb default charset=utf8;
-- 用户主机关系表
create table userandcom(
id int auto_increment primary key,
user_id int,
host_id int,
unique index(user_id,host_id),
constraint fk_key2 foreign key (user_id) references userinfo(id),
constraint fk_key3 foreign key (host_id) references computer(id)
)engine=innodb default charset=utf8;
SQL语句数据行操作补充
增
-- 增加单条数据
insert into t1 (name) values('ddd')；
增加多条数据
-- insert into t1 (name) values('ddd'),('eee');
-- 从一个表中添加另一个内容
insert into t4(name) select name from t1;
+------+------+
| id | name |
+------+------+
| NULL | aaa |
| NULL | aaa |
| NULL | ccc |
| NULL | ddd |
| NULL | eee |
+------+------+
这里出现null的原因是在创建表的时候没有添加自增和主键
在调试中发现char后面不加长度，默认的长度是1，所以要添加一个长度。这个是根据需求
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='a'
改
update tb12 set name='a' where id>12 and name='xx'
update tb12 set name='a',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;
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
通配符的意识替换的意思
%能够替换多个字符
_只能替换一个字符
select * from tb12 where name like "a%"
select * from tb12 where name like "aa_"
- 分页
select * from tb12 limit 10;
select * from tb12 limit 0,10;
select * from tb12 limit 10,10;
select * from tb12 limit 20,10;
后期的Python应用
# page = input('请输入要查看的页码')
# page = int(page)
# (page-1) * 10
# select * from tb12 limit 0,10; 第一页1
# select * from tb12 limit 10,10;第二页2
- 排序
select * from tb12 order by id desc; 大到小
select * from tb12 order by id asc; 小到大
select * from tb12 order by age desc,id desc; # 这是优先级 先按照age倒序，后按照id排序（ID中有相同的）
取后10条数据：先倒序后去取
select * from tb12 order by id desc limit 10;
mysql> select * from t5 order by name desc,id desc;
+----+------+
| id | name |
+----+------+
| 5 | eee |
| 4 | ddd |
| 3 | ccc |
| 2 | aaa |
| 1 | aaa |
+----+------+
5 rows in set (0.00 sec)
mysql> select * from t5 order by name desc,id asc;
+----+------+
| id | name |
+----+------+
| 5 | eee |
| 4 | ddd |
| 3 | ccc |
| 1 | aaa |
| 2 | aaa |
+----+------+
- 分组
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 ,不能使用where
select count(id),part_id from userinfo5 group by part_id having count(id) > 1;
- 连表操作
连表操作主要是把两张表显示在一张表上，主要用过join
select * from userinfo5,department5 -- 这种是笛卡尔积的形式 即所有的乘积
select * from userinfo5,department5 where userinfo5.part_id = department5.id;
左边全部显示
select * from userinfo5 left join department5 on userinfo5.part_id = department5.id
右边全部显示
select * from userinfo5 right join department5 on userinfo5.part_id = department5.id
select * from department5 left join userinfo5 on userinfo5.part_id = department5.id;这种就是变相的right
如果一张表显示全部，但是另一张表还有多的内容的时候，就会出现空null
inner join 将出现null时一行隐藏
select * from userinfo5 innder join department5 on userinfo5.part_id = department5.id
mysql> select * from t1 left join t5 on t1.id=t5.id;
+----+-------+------+------+
| id | name | id | name |
+----+-------+------+------+
| 1 | aaa | 1 | aaa |
| 2 | aaa | 2 | aaa |
| 3 | ccc | 3 | ccc |
| 4 | ddd | 4 | ddd |
| 5 | eee | 5 | eee |
| 6 | hahah | NULL | NULL |
+----+-------+------+------+
隐藏空行
mysql> select * from t1 inner join t5 on t1.id=t5.id;
+----+------+----+------+
| id | name | id | name |
+----+------+----+------+
| 1 | aaa | 1 | aaa |
| 2 | aaa | 2 | aaa |
| 3 | ccc | 3 | ccc |
| 4 | ddd | 4 | ddd |
| 5 | eee | 5 | eee |
+----+------+----+------+
5 rows in set (0.00 sec)
数据库的备份
数据库导出
-
mysqldump -u用户名 -p密码 数据库名称 >导出文件路径 # 结构+数据（导入的时候会自动穿件表并把表的内容插入）
-
mysqldump -u用户名 -p密码 -d 数据库名称 >导出文件路径 # 仅仅结构
导入现有数据库数据： -
mysql -uroot -p密码 数据库名称 < 文件路径
注意的是导入的时候不能用dump