mysql约束条件,修改表结构，mysql键值

一.约束条件：限制给字段赋值：null(空)，键值，默认值，额外值

mysql> create database db2; #建立db2库

mysql> use db2 #进入db2库
mysql> create table t1( #建立t1表
-> class char(9),
-> name char(10) not null , #限制name 字段不能为null
-> age tinyint not null default 19, ##限制age字段不能为null ,而且不赋值是默认是19
-> likes set ("a","b","c","d") default "a,b"
-> );
mysql> desc t1; # 查看表结构

+-------+------+-----+-------+
| class | name | age | likes |
+-------+------+-----+-------+
| NULL | bob | 29 | c,d |
+-------+------+-----+-------+

mysql> insert into t1 (class,name) values ("nsd1902","tom"); #当只是给 class,name 赋值是，其他的则给默认值
mysql> select * from t1;
+---------+------+-----+-------+
| class | name | age | likes |
+---------+------+-----+-------+
| NULL | bob | 29 | c,d |
| nsd1902 | tom | 19 | a,b |
+---------+------+-----+-------+

mysql> insert into t1 values (null,null,null,null); #报错因为第2列和第3列不允许赋空值；
ERROR 1048 (23000): Column 'name' cannot be null

mysql> insert into t1 values (null,"",12,null); # 零个字符，也不是空
mysql> select * from t1;
+---------+------+-----+-------+
| class | name | age | likes |
+---------+------+-----+-------+
| NULL | bob | 29 | c,d |
| nsd1902 | tom | 19 | a,b |
| NULL | | 12 | NULL |
+---------+------+-----+-------+

mysql> insert into t1 values (null,"null",12,null); #null里面加了“”双引号，表示普通字符；
Query OK, 1 row affected (0.07 sec)

mysql> select * from t1;
+---------+------+-----+-------+
| class | name | age | likes |
+---------+------+-----+-------+
| NULL | bob | 29 | c,d |
| nsd1902 | tom | 19 | a,b |
| NULL | | 12 | NULL |
| NULL | null | 12 | NULL |
+---------+------+-----+-------+

mysql> create table t2 (

class char(9) default "", #默认值为空字符；

name char(10) not null,

age tinyint not null default 19,

likes set("a","b","c","d") default "a,b"

);

mysql> desc t2;
+-------+----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------------------+------+-----+---------+-------+
| class | char(9) | YES | | | |
| name | char(10) | NO | | NULL | |
| age | tinyint(4) | NO | | 19 | |
| likes | set('a','b','c','d') | YES | | a,b | |
+-------+----------------------+------+-----+---------+-------+

2修改表结构（add 添加字段，modify 修改字段类型，change 修改字段名，drop 删除字段，rename 修改表名）

alter table 表名 add/modify 新字段

添加

mysql> select * from t5;
+------+----------+------+
| name | likes | sex
+------+----------+------+-
| bob | eat,game | boy
+------+----------+------+

mysql> alter table t5 add mail varchar(50) default "[email protected]"; #添加mail字段，会在表里内容添加mail字段
mysql> select * from t5;
+------+----------+------+-------------+
| name | likes | sex | mail |
+------+----------+------+-------------+
| bob | eat,game | boy | [email protected] |
+------+----------+------+-------------+

mysql> alter table t5 add qq char(11) ,add tel char(11); # 添加的新字段不赋与默认值时，给的NULL
mysql> select * from t5;
+------+----------+------+-------------+------+------+
| name | likes | sex | mail | qq | tel |
+------+----------+------+-------------+------+------+
| bob | eat,game | boy | [email protected] | NULL | NULL |
+------+----------+------+-------------+------+------+

mysql> alter table t5 add stu_num char(9) first;
mysql> select * from t5;
+---------+------+----------+------+-------------+------+------+
| stu_num | name | likes | sex | mail | qq | tel |
+---------+------+----------+------+-------------+------+------+
| NULL | bob | eat,game | boy | [email protected] | NULL | NULL |
+---------+------+----------+------+-------------+------+------+

mysql> alter table t5 add pay float(7,2) default 20000 after name; # 将新加的pay字段放在name字段的后面
mysql> select * from t5;
+---------+------+----------+----------+------+-------------+------+------+
| stu_num | name | pay | likes | sex | mail | qq | tel |
+---------+------+----------+----------+------+-------------+------+------+
| NULL | bob | 20000.00 | eat,game | boy | [email protected] | NULL | NULL |
+---------+------+----------+----------+------+-------------+------+------+

修改字段类型：（不能与已有的字段冲突）

alter table 表名 modify 字段名 char(11) not null; # 字段名后面接需要修改的字段类型，其他不变的也需要写进去

修改字段名 change

格式： alter table 表名 change 旧字段名新字段名

删除字段名 drop

格式： alter table 表名 drop 字段名（需要删除的）

mysql> alter table t5 drop sex,drop likes;

修改表名 rename

格式： alter table 源表名 rename 新表名

mysql> alter table t5 rename student_info;
mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| 学生信息 |
| a |
| student_info |
| t1 |
| t2 |
| t3 |
| t4 |
| t6 |
| t7 |
+---------------+

3 Mysql 键值

每种键都对表记录有插入限制，且有各自的功能

种类：

普通索引：index *

唯一索引 :unique

主键 : primary key *

外键: foreign key *

全文索引 fulltext

索引的定义：索引是对记录集多个字段进行排序的方法

索引的类型包括：Btree（二叉树） B+tree hash #排列的方式

索引的优缺点：

优点：加快数据的检索速度

缺点：占用物理空间，降低了数据的维护速度

普通引索index :

特点：一个表中可以给多个index字段，字段的值允许有重复，且可以赋NULL值，index字段的key标志是mul

排队信息 /var/lib/mysql/db1/t1.frm t1.idb

查看索引信息： show index from 表名\G

索引名字 key_name

删除索引： drop index 索引名 on 表名

mysql> use db2 # 进入db2库里
mysql> create table t3( name char(10), age tinyint, sex enum ("m","w"), index(name), index(age) ); #建立表
mysql> desc t3;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| name | char(10) | YES | MUL | NULL | |
| age | tinyint(4) | YES | MUL | NULL | |
| sex | enum('m','w') | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+

mysql> show index from t3\G; # 查看索引信息
mysql> drop index name on t3; # 在t3库里删除name 字段里的索引
mysql> show index from t3\G;

在已有表里面设置INDEX字段

格式：create index 索引名 on db2.t1(字段名)

mysql> create index aaa on db2.t1(name); # db2.t1里name 字段里，添加索引index，索引名字为aaa

mysql> create index age on db2.t1(age); #
mysql> desc t1;
+-------+----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------------------+------+-----+---------+-------+
| class | char(9) | YES | | NULL | |
| name | char(10) | NO | MUL | NULL | |
| age | tinyint(4) | NO | MUL | 19 | |
| likes | set('a','b','c','d') | YES | | a,b | |
+-------+----------------------+------+-----+---------+-------+
mysql> show index from db2.t1\G;

primary key 主键：使用规则查看创建删除

mysql> create table t4(
-> class char(7),
-> name char(10),
-> age tinyint,
-> stu_id char(9) primary key # 格式1
-> );
mysql> create table t5( class char(7), name char(10), age tinyint, stu_id char(9), primary key(stu_id) ); #格式2
mysql> desc t4;desc t5; #查看表结构
mysql> insert into t4 value ("nsd1902","lucy","16","nsd190208"); #添加表内容
mysql> insert into t4 value ("nsd1902","lilei","21","nsd190208"); #拥有primary key的字段的不允许重复
ERROR 1062 (23000): Duplicate entry 'nsd190208' for key 'PRIMARY'
mysql> insert into t4 value ("nsd1902","lilei","21",null); #拥有primary key的字段的不允许重复，不可以赋值为null
ERROR 1048 (23000): Column 'stu_id' cannot be null
mysql> insert into t4 value ("nsd1902","lilei","21","nsd190220");
Query OK, 1 row affected (0.06 sec)

在已有表中的设置primary key 字段:

alter table 表名 add primary key(字段名);

mysql> alter table db2.t1 add primary key(class); # 在db2库里面的t1表里面的class 字段添加 primary key;

复合主键的作用：

所有字段不同时一样就说明不重复，防止在特殊字符里面有1，2两个是重复的而导致不能写入，所有字段里面的内容是不能重复的，如果重复了就不能写入

mysql> create table t6 (
-> class char(7),
-> name char(15),
-> pay enum("yes","no") default "no",
-> primary key (class,name,pay)
-> );
mysql> desc t6;
+-------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| class | char(7) | NO | PRI | NULL | |
| name | char(15) | NO | PRI | NULL | |
| pay | enum('yes','no') | NO | PRI | no | |
+-------+------------------+------+-----+---------+-------+

mysql> insert into t6 values ("nsd1902","longlog","yes");
Query OK, 1 row affected (0.04 sec)
mysql> insert into t6 values ("nsd1902","longlog","yes"); # 三个字段同时一样是不能创建的
ERROR 1062 (23000): Duplicate entry 'nsd1902-longlog-yes' for key 'PRIMARY'
mysql> insert into t6 values ("nsd1902","longlog","no"); # 可以创建
mysql> select * from t6;
+---------+---------+-----+
| class | name | pay |
+---------+---------+-----+
| nsd1902 | longlog | yes |
| nsd1902 | longlog | no |
+---------+---------+-----+

mysql> alter table db2.t6 drop primary key; # 删除复合主键
mysql> desc t6;

mysql> alter table db2.t6 add primary key(class,name,pay)；#添加复合主键
mysql> desc t6;

通常和 AUTO_INCREMENT连用：让字段的值自增长（i++ i=i+1）

要求字段的类型是数值类型且是primary key;

mysql> select * from t7;
+----+------+------+------+---------+
| id | name | sex | age | class |
+----+------+------+------+---------+
| 1 | bob | m | 21 | nsd1902 |
| 2 | tom | m | 19 | nsd1902 | # id 是自增长的 i++
+----+------+------+------+---------+
mysql> insert into t7 values(9,"tom","m",19,"nsd1902");
mysql> insert into t7(name,sex,age,class) values("jerry","m",24,"nsd1902");
mysql> select * from t7;
+----+-------+------+------+---------+
| id | name | sex | age | class |
+----+-------+------+------+---------+
| 1 | bob | m | 21 | nsd1902 |
| 2 | tom | m | 19 | nsd1902 |
| 9 | tom | m | 19 | nsd1902 |
| 10 | jerry | m | 24 | nsd1902 | #id 由9后面添加
+----+-------+------+------+---------+
mysql> select * from t7 where id=2;

外键

定义：让当前表字段的值在另一个表中的字段值的范围内选择；

使用规则：表的存储引擎必须是innodb;

两个表中的字段类型要一致；

被参照字段必须要是索引类型的一种（primary key） # 意思是字段里面的值不能为null并且不能重复

格式： foreign key（表A的字段名）

references 表B（字段名）

on update cascade

on delete cascade

mysql> create database db3;

mysql> use db3;

mysql> create table yg(

yg_id int primary key auto_increment,

name char(15)

)engine=innodb; #添加存储引擎innodb
mysql> desc yg;

mysql> create table gz( gz_id int, gz float(7.2) default "20000", foreign key(gz_id) references yg(yg_id) on update cascade on delete cascade)engine=innodb;
Query OK, 0 rows affected (0.29 sec)
mysql> desc gz;
mysql> desc gz;

mysql> insert into yg(name) values("bob");
mysql> insert into yg(name) values("bob");

mysql> select * from yg;
+-------+------+
| yg_id | name |
+-------+------+
| 1 | bob |
| 2 | tom |
| 3 | lucy |
+-------+------+
3 rows in set (0.00 sec)
mysql> insert into gz values(4,30000);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`db3`.`gz`, CONSTRAINT `gz_ibfk_1` FOREIGN KEY (`gz_id`) REFERENCES `yg` (`yg_id`) ON DELETE CASCADE ON UPDATE CASCADE)

mysql> update yg set yg_id=8 where yg_id=2; # 验证同步更新

mysql> select * from yg;
+-------+------+
| yg_id | name |
+-------+------+
| 1 | bob |
| 3 | lucy |
| 8 | tom |
+-------+------+
3 rows in set (0.00 sec)

mysql> select * from gz;
+-------+-------+
| gz_id | gz |
+-------+-------+
| 1 | 30000 |
| 8 | 20000 |
| 3 | 20000 |
+-------+-------+

mysql> delete from yg where yg_id=8; #删除表A的字段 ,此时表B的字段也没有了

mysql> select * from yg;
+-------+------+
| yg_id | name |
+-------+------+
| 1 | bob |
| 3 | lucy |
+-------+------+
2 rows in set (0.00 sec)

mysql> select * from gz;
+-------+-------+
| gz_id | gz |
+-------+-------+
| 1 | 30000 |
| 3 | 20000 |
+-------+-------+
2 rows in set (0.00 sec)

删除外键

mysql> show create table gz\G;
*************************** 1. row ***************************
Table: gz
Create Table: CREATE TABLE `gz` (
`gz_id` int(11) DEFAULT NULL,
`gz` float DEFAULT '20000',
KEY `gz_id` (`gz_id`),
CONSTRAINT `gz_ibfk_1` FOREIGN KEY (`gz_id`) REFERENCES `yg` (`yg_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

mysql> alter table gz drop foreign key gz_ibfk_1; #删除外键
mysql> show create table gz\G;

mysql> alter table gz add foreign key(gz_id) references yg(yg_id) on update cascade on delete cascade;
mysql> alter table gz add primary key(gz_id);
mysql> show create table gz\G;

mysql约束条件,修改表结构，mysql键值

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