标签：联合 upd splay 架构 语句 ext 主键约束 bms span
为了防止不符合规范的数据进入数据库，在用户对数据进行插入、修改、删除等操作时，DBMS自动按照一定的约束条件对数据进行监测，
使不符合规范的数据不能进入数据库，以确保数据库中存储的数据正确、有效、相容。
约束条件与数据类型的宽度一样，都是可选参数，主要分为以下几种：
# NOT NULL ：非空约束，指定某列不能为空； # UNIQUE : 唯一约束，指定某列或者几列组合不能重复 # PRIMARY KEY ：主键，指定该列的值可以唯一地标识该列记录 # FOREIGN KEY ：外键，指定该行记录从属于主表中的一条记录，主要用于参照完整性
是否可空，null表示空，非字符串
not null - 不可空
null - 可空
mysql> create table t12 (id int not null); Query OK, 0 rows affected (0.02 sec) mysql> select * from t12; Empty set (0.00 sec) mysql> desc t12; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | +-------+---------+------+-----+---------+-------+ row in set (0.00 sec) #不能向id列插入空元素。 mysql> insert into t12 values (null); ERROR 1048 (23000): Column ‘id‘ cannot be null mysql> insert into t12 values (1); Query OK, 1 row affected (0.01 sec) not null示例
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
mysql> create table t13 (id1 int not null,id2 int not null default 222); Query OK, 0 rows affected (0.01 sec) mysql> desc t13; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id1 | int(11) | NO | | NULL | | | id2 | int(11) | NO | | 222 | | +-------+---------+------+-----+---------+-------+ rows in set (0.01 sec) # 只向id1字段添加值，会发现id2字段会使用默认值填充 mysql> insert into t13 (id1) values (111); Query OK, 1 row affected (0.00 sec) mysql> select * from t13; +-----+-----+ | id1 | id2 | +-----+-----+ | 111 | 222 | +-----+-----+ row in set (0.00 sec) # id1字段不能为空，所以不能单独向id2字段填充值； mysql> insert into t13 (id2) values (223); ERROR 1364 (HY000): Field ‘id1‘ doesn‘t have a default value # 向id1，id2中分别填充数据，id2的填充数据会覆盖默认值 mysql> insert into t13 (id1,id2) values (112,223); Query OK, 1 row affected (0.00 sec) mysql> select * from t13; +-----+-----+ | id1 | id2 | +-----+-----+ | 111 | 222 | | 112 | 223 | +-----+-----+ rows in set (0.00 sec) not null + default 示例
唯一约束，指定某列或者几列组合不能重复
方法一： create table department1( id int, name varchar(20) unique, comment varchar(100) ); 方法二： create table department2( id int, name varchar(20), comment varchar(100), unique(name) ); mysql> insert into department1 values(1,‘IT‘,‘技术‘); Query OK, 1 row affected (0.00 sec) mysql> insert into department1 values(1,‘IT‘,‘技术‘); ERROR 1062 (23000): Duplicate entry ‘IT‘ for key ‘name‘ unique示例
mysql> create table t1(id int not null unique); Query OK, 0 rows affected (0.02 sec) mysql> desc t1; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | +-------+---------+------+-----+---------+-------+ row in set (0.00 sec) not null 和unique的结合
create table service( id int primary key auto_increment, name varchar(20), host varchar(15) not null, port int not null, unique(host,port) #联合唯一 ); mysql> insert into service values -> (1,‘nginx‘,‘192.168.0.10‘,80), -> (2,‘haproxy‘,‘192.168.0.20‘,80), -> (3,‘mysql‘,‘192.168.0.30‘,3306) -> ; Query OK, 3 rows affected (0.01 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> insert into service(name,host,port) values(‘nginx‘,‘192.168.0.10‘,80); ERROR 1062 (23000): Duplicate entry ‘192.168.0.10-80‘ for key ‘host‘ 联合唯一
主键为了保证表中的每一条数据的该字段都是表格中的唯一值。换言之，它是用来独一无二地确认一个表格中的每一行数据。
主键可以包含一个字段或多个字段。当主键包含多个栏位时，称为组合键 (Composite Key),也可以叫联合主键。
主键可以在建置新表格时设定 (运用 CREATE TABLE 语句)，或是以改变现有的表格架构方式设定 (运用 ALTER TABLE)。
主键必须唯一，主键值非空；可以是单一字段，也可以是多字段组合。
============单列做主键=============== #方法一：not null+unique create table department1( id int not null unique, #主键 name varchar(20) not null unique, comment varchar(100) ); mysql> desc department1; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | NO | UNI | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.01 sec) #方法二：在某一个字段后用primary key create table department2( id int primary key, #主键 name varchar(20), comment varchar(100) ); mysql> desc department2; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.00 sec) #方法三：在所有字段后单独定义primary key create table department3( id int, name varchar(20), comment varchar(100), primary key(id); #创建主键并为其命名pk_name mysql> desc department3; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.01 sec) # 方法四：给已经建成的表添加主键约束 mysql> create table department4( -> id int, -> name varchar(20), -> comment varchar(100)); Query OK, 0 rows affected (0.01 sec) mysql> desc department4; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.01 sec) mysql> alter table department4 modify id int primary key; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc department4; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.01 sec) 单字段主键
==================多列做主键================ create table service( ip varchar(15), port char(5), service_name varchar(10) not null, primary key(ip,port) ); mysql> desc service; +--------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+-------+ | ip | varchar(15) | NO | PRI | NULL | | | port | char(5) | NO | PRI | NULL | | | service_name | varchar(10) | NO | | NULL | | +--------------+-------------+------+-----+---------+-------+ rows in set (0.00 sec) mysql> insert into service values -> (‘172.16.45.10‘,‘3306‘,‘mysqld‘), -> (‘172.16.45.11‘,‘3306‘,‘mariadb‘) -> ; Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> insert into service values (‘172.16.45.10‘,‘3306‘,‘nginx‘); ERROR 1062 (23000): Duplicate entry ‘172.16.45.10-3306‘ for key ‘PRIMARY‘ 多字段主键
#不指定id，则自动增长 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 | | +-------+-----------------------+------+-----+---------+----------------+ mysql> insert into student(name) values -> (‘egon‘), -> (‘alex‘) -> ; mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 1 | egon | male | | 2 | alex | male | +----+------+------+ #也可以指定id mysql> insert into student values(4,‘asb‘,‘female‘); Query OK, 1 row affected (0.00 sec) mysql> insert into student values(7,‘wsb‘,‘female‘); Query OK, 1 row affected (0.00 sec) mysql> select * from student; +----+------+--------+ | id | name | sex | +----+------+--------+ | 1 | egon | male | | 2 | alex | male | | 4 | asb | female | | 7 | wsb | female | +----+------+--------+ #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 mysql> delete from student; Query OK, 4 rows affected (0.00 sec) mysql> select * from student; Empty set (0.00 sec) mysql> insert into student(name) values(‘ysb‘); mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 8 | ysb | male | +----+------+------+ #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 mysql> truncate student; Query OK, 0 rows affected (0.01 sec) mysql> insert into student(name) values(‘egon‘); Query OK, 1 row affected (0.01 sec) mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 1 | egon | male | +----+------+------+ row in set (0.00 sec) 设置auto_increment
多表 ：
假设我们要描述所有公司的员工，需要描述的属性有这些 ： 工号 姓名 部门
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： 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
mysql> create table departments (dep_id int(4),dep_name varchar(11)); Query OK, 0 rows affected (0.02 sec) mysql> desc departments; +----------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+-------+ | dep_id | int(4) | YES | | NULL | | | dep_name | varchar(11) | YES | | NULL | | +----------+-------------+------+-----+---------+-------+ rows in set (0.00 sec) # 创建外键不成功 mysql> create table staff_info (s_id int,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id)); ERROR 1215 (HY000): Cannot add foreign key # 设置dep_id非空，仍然不能成功创建外键 mysql> alter table departments modify dep_id int(4) not null; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc departments; +----------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+-------+ | dep_id | int(4) | NO | | NULL | | | dep_name | varchar(11) | YES | | NULL | | +----------+-------------+------+-----+---------+-------+ rows in set (0.00 sec) mysql> create table staff_info (s_id int,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id)); ERROR 1215 (HY000): Cannot add foreign key constraint # 当设置字段为unique唯一字段时，设置该字段为外键成功 mysql> alter table departments modify dep_id int(4) unique; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc departments; +----------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+-------+ | dep_id | int(4) | YES | UNI | NULL | | | dep_name | varchar(11) | YES | | NULL | | +----------+-------------+------+-----+---------+-------+ rows in set (0.01 sec) mysql> create table staff_info (s_id int,name varchar(20),dep_id int,foreign key(dep_id) references departments(dep_id)); Query OK, 0 rows affected (0.02 sec) 创造外键的条件
#表类型必须是innodb存储引擎，且被关联的字段，即references指定的另外一个表的字段，必须保证唯一 create table department( id int primary key, name varchar(20) not null )engine=innodb; #dpt_id外键，关联父表（department主键id），同步更新，同步删除 create table employee( id int primary key, name varchar(20) not null, dpt_id int, foreign key(dpt_id) references department(id) on delete cascade # 连级删除 on update cascade # 连级更新 )engine=innodb; #先往父表department中插入记录 insert into department values (1,‘教质部‘), (2,‘技术部‘), (3,‘人力资源部‘); #再往子表employee中插入记录 insert into employee values (1,‘yuan‘,1), (2,‘nezha‘,2), (3,‘egon‘,2), (4,‘alex‘,2), (5,‘wusir‘,3), (6,‘李沁洋‘,3), (7,‘皮卡丘‘,3), (8,‘程咬金‘,3), (9,‘程咬银‘,3) ; #删父表department，子表employee中对应的记录跟着删 mysql> delete from department where id=2; Query OK, 1 row affected (0.00 sec) mysql> select * from employee; +----+-----------+--------+ | id | name | dpt_id | +----+-----------+--------+ | 1 | yuan | 1 | | 5 | wusir | 3 | | 6 | 李沁洋 | 3 | | 7 | 皮卡丘 | 3 | | 8 | 程咬金 | 3 | | 9 | 程咬银 | 3 | +----+-----------+--------+ rows in set (0.00 sec) #更新父表department，子表employee中对应的记录跟着改 mysql> update department set id=2 where id=3; Query OK, 1 row affected (0.01 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from employee; +----+-----------+--------+ | id | name | dpt_id | +----+-----------+--------+ | 1 | yuan | 1 | | 5 | wusir | 2 | | 6 | 李沁洋 | 2 | | 7 | 皮卡丘 | 2 | | 8 | 程咬金 | 2 | | 9 | 程咬银 | 2 | +----+-----------+--------+ rows in set (0.00 sec) 外键操作示例
标签：联合 upd splay 架构 语句 ext 主键约束 bms span
原文地址：https://www.cnblogs.com/yidashi110/p/9756342.html
