約束條件與數據類型的寬度是一樣的，都是可選參數。
作用：用於保證數據的完整性和一致性。
主要分為：
PRIMARY KEY (PK) #標識該字段為該表的主鍵，可以唯一的標識記錄 FOREIGN KEY (FK) #標識該字段為該表的外鍵 NOT NULL #標識該字段不能為空 UNIQUE KEY (UK) #標識該字段的值是唯一的 AUTO_INCREMENT #標識該字段的值自動增長（整數類型，而且為主鍵） DEFAULT #為該字段設置默認值 UNSIGNED #無符號 ZEROFILL #使用0填充
說明：
#1. 是否允許為空，默認NULL，可設置NOT NULL，字段不允許為空，必須賦值 #2. 字段是否有默認值，缺省的默認值是NULL，如果插入記錄時不給字段賦值，此字段使用默認值 sex enum('male','female') not null default 'male' #必須為正值（無符號） 不允許為空 默認是20 age int unsigned NOT NULL default 20 # 3. 是否是key 主鍵 primary key 外鍵 foreign key 索引 (index,unique...)
not null 與 default:
是否可空，null表示空，非字符串
not null 不可為空；
null 可空；
默認值，創建列時可以指定默認值，當插入數據時如果未主動設置，則自動添加默認值。
create table tb1(id int not null default 2,num int not null);
驗證：
mysql> create table t11(id int);# id字段默認可以為空 Query OK, 0 rows affected (0.05 sec) mysql> desc t11; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.03 sec) mysql> insert into t11 values(); #給t11表插一個空的值 Query OK, 1 row affected (0.00 sec) #查詢結果如下 mysql> select * from t11; +------+ | id | +------+ | NULL | +------+ 1 row in set (0.00 sec)
驗證2：
mysql> create table t12(id int not null);#設置字段id不為空 Query OK, 0 rows affected (0.03 sec) mysql> desc t12; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.01 sec) mysql> insert into t12 values();#不能插入空 ERROR 1364 (HY000): Field 'id' doesn't have a default value
驗證3：
# 第一種情況 mysql> create table t13(id int default 1); Query OK, 0 rows affected (0.03 sec) mysql> desc t13; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | YES | | 1 | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.01 sec) mysql> insert into t13 values(); Query OK, 1 row affected (0.00 sec) mysql> select * from t13; +------+ | id | +------+ | 1 | +------+ 1 row in set (0.00 sec) # 第二種情況 mysql> create table t14(id int not null default 2); Query OK, 0 rows affected (0.02 sec) mysql> desc t14; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | | 2 | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.01 sec) mysql> select * from t14; +----+ | id | +----+ | 2 | +----+ 1 row in set (0.00 sec)
unique:
單例唯一。
#第一種創建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'); #第二種創建unique的方式 create table department( id int, name char(10) , unique(id), unique(name) ); insert into department values(1,'it'),(2,'sale');
聯合唯一：
# 創建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 | | +-------+----------+------+-----+---------+-------+ 3 rows in set (0.01 sec) #聯合唯一，只要兩列記錄，有一列不同，既符合聯合唯一的約束 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 | +------+---------------+------+ 3 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'
primary key(主鍵):
一個表種可以：單列做主鍵，多列做主鍵(復合主鍵)。
約束：等價於 not null unique，字段的值不為空，且唯一。
存儲引擎默認是(innodb)：對於innodb存儲引擎來說，一張表必須有一個主鍵。
單列主鍵：
# 創建t14表，為id字段設置主鍵，唯一的不同的記錄 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的化學反應，相當於給id設置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 | | +-------+----------+------+-----+---------+-------+ 2 rows in set (0.02 sec)
復合主鍵：
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);
auto_increment:
約束：約束的字段為自動增長，約束的字段必須同時被key約束。
# 創建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 | | +-------+-----------------------+------+-----+---------+----------------+ 3 rows in set (0.17 sec) #插入記錄 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 | +----+--------+------+ 2 rows in set (0.00 sec)
雖然是自增長，但是也可以指定id: 且不能插入比最后一條記錄的id小的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 (0.01 sec) mysql> select * from student; +----+--------+--------+ | id | name | sex | +----+--------+--------+ | 1 | 老白 | male | | 2 | 小白 | male | | 4 | asb | female | | 7 | wsb | female | +----+--------+--------+ 4 rows in set (0.00 sec) # 再次插入一條不指定id的記錄，會在之前的最后一條記錄繼續增長 mysql> insert into student(name) values ('大白'); Query OK, 1 row affected (0.00 sec) mysql> select * from student; +----+--------+--------+ | id | name | sex | +----+--------+--------+ | 1 | 老白 | male | | 2 | 小白 | male | | 4 | asb | female | | 7 | wsb | female | | 8 | 大白 | male | +----+--------+--------+ 5 rows in set (0.00 sec)
對於自增的字段，再用delete刪除后，在插入值，該字段扔按照刪除前的位置繼續增長。
mysql> delete from student; Query OK, 5 rows affected (0.00 sec) mysql> select * from student; Empty set (0.00 sec) mysql> select * from student; Empty set (0.00 sec) mysql> insert into student(name) values('ysb'); Query OK, 1 row affected (0.01 sec) mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 9 | ysb | male | +----+------+------+ 1 row in set (0.00 sec) #應該用truncate清空表，比起delete一條一條地刪除記錄，truncate是直接清空表，在刪除大表時用它 mysql> truncate student; Query OK, 0 rows affected (0.03 sec) mysql> insert into student(name) values('xiaobai'); Query OK, 1 row affected (0.00 sec) mysql> select * from student; +----+---------+------+ | id | name | sex | +----+---------+------+ | 1 | xiaobai | male | +----+---------+------+ 1 row in set (0.00 sec)
auto_increment_increment 和 auto_increment_offset:
查看可用的 開頭auto_inc的詞 mysql> show variables like 'auto_inc%'; +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 1 | | auto_increment_offset | 1 | +--------------------------+-------+ 2 rows in set (0.02 sec) # 步長auto_increment_increment,默認為1 # 起始的偏移量auto_increment_offset, 默認是1 # 設置步長 為會話設置，只在本次連接中有效 set session auto_increment_increment=5; #全局設置步長 都有效。 set global auto_increment_increment=5; # 設置起始偏移量 set global auto_increment_offset=3; #強調：If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored. 翻譯：如果auto_increment_offset的值大於auto_increment_increment的值，則auto_increment_offset的值會被忽略 # 設置完起始偏移量和步長之后，再次執行show variables like'auto_inc%'; 發現跟之前一樣，必須先exit,再登錄才有效。 mysql> show variables like'auto_inc%'; +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 5 | | auto_increment_offset | 3 | +--------------------------+-------+ 2 rows in set (0.00 sec) #因為之前有一條記錄id=1 mysql> select * from student; +----+---------+------+ | id | name | sex | +----+---------+------+ | 1 | xiaobai | male | +----+---------+------+ 1 row in set (0.00 sec) # 下次插入的時候，從起始位置3開始，每次插入記錄id+5 mysql> insert into student(name) values('ma1'),('ma2'),('ma3'); Query OK, 3 rows affected (0.00 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> select * from student; +----+---------+------+ | id | name | sex | +----+---------+------+ | 1 | xiaobai | male | | 3 | ma1 | male | | 8 | ma2 | male | | 13 | ma3 | male | +----+---------+------+
清空表區分delete和truncate的區別：
delete：如果有自增id,無論刪除的是否是最后一條記錄，下一次的插入記錄，都會以之前最后一條記錄的id自增。
truncate 表名：刪除表中的數據，但是保留了表的結構，下一次插入記錄，會從1開始(或者是設置的起始id)。
foreign key:
事例：公司有3個部門，但是有1個億的員工，那意味着部門這個字段需要重復存儲，部門名字越長，越浪費。
解決方法：我們完全可以定義一個部門表，然后讓員工信息表關聯該表，如何關聯，即foreign key。
此時有兩張表，一張是employee表,簡稱emp表（關聯表，也就從表，擁有外鍵的表）。一張是department表，簡稱dep表（被關聯表，也叫主表）。
創建兩張表：
#1.創建表時先創建被關聯表，再創建關聯表 # 先創建被關聯表（dep表） create table dep( id int primary key, name varchar(20) not null, descripe varchar(20) not null ); #再創建關聯表（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.插入記錄時，先往被關聯表中插入記錄，再往關聯表中插入記錄 insert into dep values (1,'IT','IT技術有限部門'), (2,'銷售部','銷售部門'), (3,'財務部','花錢太多部門'); 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.刪除表 #按道理來說，刪除了部門表中的某個部門，員工表的有關聯的記錄相繼刪除。 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`)) #但是先刪除員工表的記錄之后，再刪除當前部門就沒有任何問題 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 | +----+----------+-----+--------+ 4 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技術有限部門 | | 2 | 銷售部 | 銷售部門 | +----+-----------+----------------------+ 2 rows in set (0.00 sec)
上面的刪除表記錄的操作比較繁瑣，按道理講，裁掉一個部門，該部門的員工也會被裁掉。其實呢，在建表的時候還有個很重要的內容，叫同步刪除，同步更新。
需要在關聯表中加入：
on delete cascade(同步刪除)
on update cascade(同步更新)：
將emp表修改:
create table emp( id int primary key, name varchar(20) not null, age int not null, dep_id int, dforeign key(dep_id) references dep(id) on delete cascade #同步刪除 on update cascade #同步更新 );
從表中刪除數據，才會有同步事件：
#再去刪被關聯表(dep)的記錄，關聯表(emp)中的記錄也跟着刪除 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技術有限部門 | | 2 | 銷售部 | 銷售部門 | +----+-----------+----------------------+ 2 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 | +----+----------+-----+--------+ 4 rows in set (0.00 sec) #再去更改被關聯表(dep)的記錄，關聯表(emp)中的記錄也跟着更改 mysql> update dep set id=222 where id=2; Query OK, 1 row affected (0.02 sec) Rows matched: 1 Changed: 1 Warnings: 0 # 趕緊去查看一下兩張表是否都被刪除了，是否都被更改了 mysql> select * from dep; +-----+-----------+----------------------+ | id | name | descripe | +-----+-----------+----------------------+ | 1 | IT | IT技術有限部門 | | 222 | 銷售部 | 銷售部門 | +-----+-----------+----------------------+ 2 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 | +----+----------+-----+--------+ 4 rows in set (0.00 sec)