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基本介绍
PRIMARY KEY (PK) #标识该字段为该表的主键，可以唯一的标识记录 FOREIGN KEY (FK) #标识该字段为该表的外键 NOT NULL #标识该字段不能为空 UNIQUE KEY (UK) #标识该字段的值是唯一的 AUTO_INCREMENT #标识该字段的值自动增长（整数类型，而且为主键） DEFAULT #为该字段设置默认值 UNSIGNED #无符号 ZEROFILL #使用0填充 #===========说明=============== #1. 是否允许为空，默认NULL，可设置NOT NULL，字段不允许为空，必须赋值 #2. 字段是否有默认值，缺省的默认值是NULL，如果插入记录时不给字段赋值，此字段使用默认值 sex enum('male','female') not null default 'male' #必须为正值（无符号） 不允许为空 默认是20 age int unsigned NOT NULL default 20 # 3. 是否是key 主键 primary key 外键 foreign key 索引 (index,unique...) -
not nul 与 default
# 是否可空，null表示空，非字符串 # not null - 不可空 # null - 可空 # 默认值，创建列时可以指定默认值，当插入数据时如果未主动设置，则自动添加默认值 create table tb1( nid int not null defalut 2, num int not null );
验证1 mysql> create table t11(id int);# id字段默认可以为空 2 Query OK, 0 rows affected (0.05 sec) 3 4 mysql> desc t11; 5 +-------+---------+------+-----+---------+-------+ 6 | Field | Type | Null | Key | Default | Extra | 7 +-------+---------+------+-----+---------+-------+ 8 | id | int(11) | YES | | NULL | | 9 +-------+---------+------+-----+---------+-------+ 10 row in set (0.03 sec) 11 mysql> insert into t11 values(); #给t11表插一个空的值 12 Query OK, 1 row affected (0.00 sec) 13 14 #查询结果如下 15 mysql> select * from t11; 16 +------+ 17 | id | 18 +------+ 19 | NULL | 20 +------+ 21 row in set (0.00 sec) 22 23 默认值可以为空
设置not null，插入值时不能为空1 mysql> create table t12(id int not null);#设置字段id不为空 2 Query OK, 0 rows affected (0.03 sec) 3 4 mysql> desc t12; 5 +-------+---------+------+-----+---------+-------+ 6 | Field | Type | Null | Key | Default | Extra | 7 +-------+---------+------+-----+---------+-------+ 8 | id | int(11) | NO | | NULL | | 9 +-------+---------+------+-----+---------+-------+ 10 row in set (0.01 sec) 11 12 mysql> insert into t12 values();#不能插入空 13 ERROR 1364 (HY000): Field 'id' doesn't have a default value 14 15 设置not null，插入值时不能为空
设置id字段有默认值后，则无论id字段是null还是not null，都可以插入空，插入空默认填入default指定的默认值1 # 第一种情况 2 mysql> create table t13(id int default 1); 3 Query OK, 0 rows affected (0.03 sec) 4 5 mysql> desc t13; 6 +-------+---------+------+-----+---------+-------+ 7 | Field | Type | Null | Key | Default | Extra | 8 +-------+---------+------+-----+---------+-------+ 9 | id | int(11) | YES | | 1 | | 10 +-------+---------+------+-----+---------+-------+ 11 row in set (0.01 sec) 12 13 mysql> insert into t13 values(); 14 Query OK, 1 row affected (0.00 sec) 15 16 mysql> select * from t13; 17 +------+ 18 | id | 19 +------+ 20 | 1 | 21 +------+ 22 row in set (0.00 sec) 23 24 25 # 第二种情况 26 mysql> create table t14(id int not null default 2); 27 Query OK, 0 rows affected (0.02 sec) 28 29 mysql> desc t14; 30 +-------+---------+------+-----+---------+-------+ 31 | Field | Type | Null | Key | Default | Extra | 32 +-------+---------+------+-----+---------+-------+ 33 | id | int(11) | NO | | 2 | | 34 +-------+---------+------+-----+---------+-------+ 35 row in set (0.01 sec) 36 37 mysql> select * from t14; 38 +----+ 39 | id | 40 +----+ 41 | 2 | 42 +----+ 43 row in set (0.00 sec) 44 45 设置id字段有默认值后，则无论id字段是null还是not null，都可以插入空，插入空默认填入default指定的默认值
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unique
- 单列唯一
#使用约束条件unique，来对公司部门的字段进行设置。 #第一种创建unique的方式 #例子1： create table department( id int, name char(10) unique ); mysql> insert into department values(1,'it'),(2,'it'); ERROR 1062 (23000): Duplicate entry 'it' for key 'name' #例子2： create table department( id int unique, name char(10) unique ); insert into department values(1,'it'),(2,'sale'); #第二种创建unique的方式 create table department( id int, name char(10) , unique(id), unique(name) ); insert into department values(1,'it'),(2,'sale'); - 联合唯一
# 创建services表 mysql> create table services( id int, ip char(15), port int, unique(id), unique(ip,port) ); Query OK, 0 rows affected (0.05 sec) mysql> desc services; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | YES | UNI | NULL | | | ip | char(15) | YES | MUL | NULL | | | port | int(11) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ rows in set (0.01 sec) #联合唯一，只要两列记录，有一列不同，既符合联合唯一的约束 mysql> insert into services values (1,'192,168,11,23',80), (2,'192,168,11,23',81), (3,'192,168,11,25',80); Query OK, 3 rows affected (0.01 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> select * from services; +------+---------------+------+ | id | ip | port | +------+---------------+------+ | 1 | 192,168,11,23 | 80 | | 2 | 192,168,11,23 | 81 | | 3 | 192,168,11,25 | 80 | +------+---------------+------+ rows in set (0.00 sec) mysql> insert into services values (4,'192,168,11,23',80); ERROR 1062 (23000): Duplicate entry '192,168,11,23-80' for key 'ip'
- 单列唯一
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primary key
介绍- 单列主键
# 创建t14表，为id字段设置主键，唯一的不同的记录 create table t14( id int primary key, name char(16) ); insert into t14 values (1,'xiaoma'), (2,'xiaohong'); mysql> insert into t14 values(2,'wxxx'); ERROR 1062 (23000): Duplicate entry '6' for key 'PRIMARY' # not null + unique的化学反应，相当于给id设置primary key create table t15( id int not null unique, name char(16) ); mysql> create table t15( -> id int not null unique, -> name char(16) -> ); Query OK, 0 rows affected (0.01 sec) mysql> desc t15; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | char(16) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ rows in set (0.02 sec) - 复合主键
create table t16( ip char(15), port int, primary key(ip,port) ); insert into t16 values ('1.1.1.2',80), ('1.1.1.2',81); 验证复合主键的使用
- 单列主键
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auto_increment
#约束:约束的字段为自动增长，约束的字段必须同时被key约束
# 创建student create table student( id int primary key auto_increment, name varchar(20), sex enum('male','female') default 'male' ); mysql> desc student; +-------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | YES | | NULL | | | sex | enum('male','female') | YES | | male | | +-------+-----------------------+------+-----+---------+----------------+ rows in set (0.17 sec) #插入记录 mysql> insert into student(name) values ('老白'),('小白'); Query OK, 2 rows affected (0.01 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> select * from student; +----+--------+------+ | id | name | sex | +----+--------+------+ | 1 | 老白 | male | | 2 | 小白 | male | +----+--------+------+ rows in set (0.00 sec) 不指定id，则自动增长 -
也可以指定id
1 mysql> insert into student values(4,'asb','female'); 2 Query OK, 1 row affected (0.00 sec) 3 4 mysql> insert into student values(7,'wsb','female'); 5 Query OK, 1 row affected (0.01 sec) 6 7 mysql> select * from student; 8 +----+--------+--------+ 9 | id | name | sex | 10 +----+--------+--------+ 11 | 1 | 老白 | male | 12 | 2 | 小白 | male | 13 | 4 | asb | female | 14 | 7 | wsb | female | 15 +----+--------+--------+ 16 rows in set (0.00 sec) 17 18 # 再次插入一条不指定id的记录，会在之前的最后一条记录继续增长 19 mysql> insert into student(name) values ('大白'); 20 Query OK, 1 row affected (0.00 sec) 21 22 mysql> select * from student; 23 +----+--------+--------+ 24 | id | name | sex | 25 +----+--------+--------+ 26 | 1 | 老白 | male | 27 | 2 | 小白 | male | 28 | 4 | asb | female | 29 | 7 | wsb | female | 30 | 8 | 大白 | male | 31 +----+--------+--------+ 32 rows in set (0.00 sec) 33 34 也可以指定id
对于自增的字段，在用delete删除后，再插入值，该字段仍按照删除前的位置继续增长1 mysql> delete from student; 2 Query OK, 5 rows affected (0.00 sec) 3 4 mysql> select * from student; 5 Empty set (0.00 sec) 6 7 mysql> select * from student; 8 Empty set (0.00 sec) 9 10 mysql> insert into student(name) values('ysb'); 11 Query OK, 1 row affected (0.01 sec) 12 13 mysql> select * from student; 14 +----+------+------+ 15 | id | name | sex | 16 +----+------+------+ 17 | 9 | ysb | male | 18 +----+------+------+ 19 row in set (0.00 sec) 20 21 #应该用truncate清空表，比起delete一条一条地删除记录，truncate是直接清空表，在删除大表时用它 22 mysql> truncate student; 23 Query OK, 0 rows affected (0.03 sec) 24 25 mysql> insert into student(name) values('xiaobai'); 26 Query OK, 1 row affected (0.00 sec) 27 28 mysql> select * from student; 29 +----+---------+------+ 30 | id | name | sex | 31 +----+---------+------+ 32 | 1 | xiaobai | male | 33 +----+---------+------+ 34 row in set (0.00 sec) 35 36 mysql> 37 38 对于自增的字段，在用delete删除后，再插入值，该字段仍按照删除前的位置继续增长
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清空表区分delete和truncate的区别：
delete from t1; #如果有自增id，新增的数据，仍然是以删除前的最后一样作为起始。
truncate table t1;数据量大，删除速度比上一条快，且直接从零开始。
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foreign key
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快速理解foreign key
之前创建表的时候都是在一张表中添加记录，比如如下表：
公司有3个部门，但是有1个亿的员工，那意味着部门这个字段需要重复存储，部门名字越长，越浪费。 - 解决方法
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我们完全可以定义一个部门表
然后让员工信息表关联该表，如何关联，即foreign key
我们可以将上表改为如下结构：
此时有两张表，一张是employee表,简称emp表（关联表，也就从表）。一张是department表，简称dep表（被关联表，也叫主表） - 创建两张表操作：
#1.创建表时先创建被关联表，再创建关联表 # 先创建被关联表（dep表） create table dep( id int primary key, name varchar(20) not null, descripe varchar(20) not null ); #再创建关联表（emp表） create table emp( id int primary key, name varchar(20) not null, age int not null, dep_id int, constraint fk_dep foreign key(dep_id) references dep(id) ); #2.插入记录时，先往被关联表中插入记录，再往关联表中插入记录 insert into dep values (1,'IT','IT技术有限部门'), (2,'销售部','销售部门'), (3,'财务部','花钱太多部门'); insert into emp values (1,'zhangsan',18,1), (2,'lisi',19,1), (3,'egon',20,2), (4,'yuanhao',40,3), (5,'alex',18,2); 3.删除表 #按道理来说，删除了部门表中的某个部门，员工表的有关联的记录相继删除。 mysql> delete from dep where id=3; ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`db5`.`emp`, CONSTRAINT `fk_name` FOREIGN KEY (`dep_id`) REFERENCES `dep` (`id`)) #但是先删除员工表的记录之后，再删除当前部门就没有任何问题 mysql> delete from emp where dep =3; Query OK, 1 row affected (0.00 sec) mysql> select * from emp; +----+----------+-----+--------+ | id | name | age | dep_id | +----+----------+-----+--------+ | 1 | zhangsan | 18 | 1 | | 2 | lisi | 18 | 1 | | 3 | egon | 20 | 2 | | 5 | alex | 18 | 2 | +----+----------+-----+--------+ rows in set (0.00 sec) mysql> delete from dep where id=3; Query OK, 1 row affected (0.00 sec) mysql> select * from dep; +----+-----------+----------------------+ | id | name | descripe | +----+-----------+----------------------+ | 1 | IT | IT技术有限部门 | | 2 | 销售部 | 销售部门 | +----+-----------+----------------------+ rows in set (0.00 sec) -
上面的删除表记录的操作比较繁琐，按道理讲，裁掉一个部门，该部门的员工也会被裁掉。其实呢，在建表的时候还有个很重要的内容，叫同步删除，同步更新
接下来将刚建好的两张表全部删除，先删除关联表（emp）,再删除被关联表（dep）
接下来：
重复上面的操作建表
注意：在关联表中加入
on delete cascade #同步删除
on update cascade #同步更新create table emp( id int primary key, name varchar(20) not null, age int not null, dep_id int, constraint fk_dep foreign key(dep_id) references dep(id) on delete cascade #同步删除 on update cascade #同步更新 ); - 接下来的操作，就复合我们正常的生活中的情况了。
#再去删被关联表(dep)的记录，关联表(emp)中的记录也跟着删除 mysql> delete from dep where id=3; Query OK, 1 row affected (0.00 sec) mysql> select * from dep; +----+-----------+----------------------+ | id | name | descripe | +----+-----------+----------------------+ | 1 | IT | IT技术有限部门 | | 2 | 销售部 | 销售部门 | +----+-----------+----------------------+ rows in set (0.00 sec) mysql> select * from emp; +----+----------+-----+--------+ | id | name | age | dep_id | +----+----------+-----+--------+ | 1 | zhangsan | 18 | 1 | | 2 | lisi | 19 | 1 | | 3 | egon | 20 | 2 | | 5 | alex | 18 | 2 | +----+----------+-----+--------+ rows in set (0.00 sec) #再去更改被关联表(dep)的记录，关联表(emp)中的记录也跟着更改 mysql> update dep set id=222 where id=2; Query OK, 1 row affected (0.02 sec) Rows matched: 1 Changed: 1 Warnings: 0 # 赶紧去查看一下两张表是否都被删除了，是否都被更改了 mysql> select * from dep; +-----+-----------+----------------------+ | id | name | descripe | +-----+-----------+----------------------+ | 1 | IT | IT技术有限部门 | | 222 | 销售部 | 销售部门 | +-----+-----------+----------------------+ rows in set (0.00 sec) mysql> select * from emp; +----+----------+-----+--------+ | id | name | age | dep_id | +----+----------+-----+--------+ | 1 | zhangsan | 18 | 1 | | 2 | lisi | 19 | 1 | | 3 | egon | 20 | 222 | | 5 | alex | 18 | 222 | +----+----------+-----+--------+ rows in set (0.00 sec)
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