1. 本节目标
掌握约束的使用场景
掌握非空约束、默认约束、主键约束、唯一约束、外键约束、CHECK的创建
2. 什么是数据库约束
数据库约束是指对数据库表中的数据所施加的规则或条件，用于确保数据的准确性和可靠性。这 些约束可以是基于数据类型、值范围、唯一性、非空等规则，以确保数据的正确性和相容性
3. 约束类型
|------------------|-------------------------------------------------------|
| 类型 | 说明 |
| NOT NULL非空约束 | 指定非空约束的列不能存储 NULL 值 |
| DEFALUT 默认约束 | 当没有给列赋值时使用的默认值 |
| UNIQUE 唯一约束 | 指定唯一约束的列每行数据必须有唯一的值 |
| PRIMARY KEY 主键约束 | NOT NULL 和 UNIQUE的结合，可以指定一个列或多个列，有助于防止数据 重复和提高数据的查询性能 |
| FOREIGN KEY 外键约束 | 外键约束是一种关系约束，用于定义两个表之间的关联关系，可以确保数据 的完整性和一致性 |
| CHECK 约束 | 用于限制列或数据在数据库表中的值，确保数据的准确性和可靠性 |
4. NOT NULL 非空约束
比如创建一个学生表，学生名为NULL时，这条记录是不完整的
此约束需要列不为空
查看表结构，NULL列为NO表示值不允许为NULL，YES表示值可以为NULL
5. DEFALUT 默认值约束
DEFAULT 约束用于向列中插入默认值，如果没有为列设置值，那么会将默认值设置到该列
重构学生表，新增年龄列
java
drop table student;
# 创建学生表，加入年龄列
create table student (
id bigint,
name varchar(20) not null,
age int
);插入一条记录，没有设置默认约束时，不指定年龄的值时列为NULL
java
insert into student(id, name) values (1, '张三');
select * from student;
+------+--------+------+
| id | name | age |
+------+--------+------+
| 1 | 张三 | NULL | # 年龄值为NULL
+------+--------+------+
1 row in set (0.00 sec)重构学生表，为年龄的列加入默认约束
java
drop table student;
# 为年龄列加入默认约束
create table student (
id bigint,
name varchar(20) not null,
age int DEFAULT 18
);插入一条记录，不指定年龄的值时列使用了默认值
java
select * from student;
+------+--------+------+
| id | name | age |
+------+--------+------+
| 1 | 张三 | 18 |
+------+--------+------+
1 row in set (0.00 sec)查看表结构，年龄列的默认值为18
java
desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | bigint | YES | | NULL | |
| name | varchar(20) | NO | | NULL | |
| age | int | YES | | 18 | |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)当手动明确指年龄列为NULL时列值为NULL
java
insert into student(id, name, age) values (2, '李四', NULL);
Query OK, 1 row affected (0.00 sec)
select * from student;
+------+--------+------+
| id | name | age |
+------+--------+------+
| 1 | 张三 | 18 |
| 2 | 李四 | NULL |
+------+--------+------+
2 rows in set (0.00 sec)6. UNIQUE 唯一约束
指定了唯一约束的列，该列的值在所有记录中不能重复，比如一个人的身份证号，学生的学号等
重构学生表，新增学号列
java
drop table student;
# 学号列设置唯一约束
create table student (
id bigint,
name varchar(20) not null,
age int DEFAULT 18,
sno varchar(10)
);不设置唯一约束时，学号可以重复
java
insert into student(id, name, sno) values (1, '张三', '100001');
Query OK, 1 row affected (0.00 sec)
insert into student(id, name, sno) values (2, '李四', '100001');
Query OK, 1 row affected (0.00 sec)
select * from student;
+------+--------+------+--------+
| id | name | age | sno |
+------+--------+------+--------+
| 1 | 张三 | 18 | 100001 |
| 2 | 李四 | 18 | 100001 |
+------+--------+------+--------+
2 rows in set (0.00 sec)重构学生表，为学号列设置唯一约束
java
drop table student;
create table student (
id bigint,
name varchar(20) not null,
age int DEFAULT 18,
sno varchar(10) UNIQUE # 唯一约束
);插入重复的学号时报错，唯一约束生效
java
insert into student(id, name, sno) values (1, '张三', '100001');
Query OK, 1 row affected (0.01 sec)
insert into student(id, name, sno) values (2, '李四', '100001');
ERROR 1062 (23000): Duplicate entry '100001' for key 'student.sno'
select * from student;
+------+--------+------+--------+
| id | name | age | sno |
+------+--------+------+--------+
| 1 | 张三 | 18 | 100001 |
+------+--------+------+--------+
1 row in set (0.00 sec)查看表结构，Key列显示UNI表示唯一约束
java
desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | bigint | YES | | NULL | |
| name | varchar(20) | NO | | NULL | |
| age | int | YES | | 18 | |
| sno | varchar(10) | YES | UNI | NULL | |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)7. PRIMARY KEY 主键约束
重构学生表，为ID列添加非空和唯一约束
java
drop table student;
create table student (
id bigint not null unique,
name varchar(20) not null,
age int DEFAULT 18,
sno varchar(10) UNIQUE
);查看表结构，添加了非空和唯一约束之后Key列显示PRI表示主键
java
desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | bigint | NO | PRI | NULL | | # 主键
| name | varchar(20) | NO | | NULL | |
| age | int | YES | | 18 | |
| sno | varchar(10) | YES | UNI | NULL | |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)当Id列的重复时会发生主键冲突
java
insert into student(id, name, sno) values (1, '张三', '100001');
Query OK, 1 row affected (0.01 sec)
insert into student(id, name, sno) values (1, '李四', '100001');
ERROR 1062 (23000): Duplicate entry '1' for key 'student.id'通常把主键列设置为自动增长，让数据库维护主键值
java
drop table student;
# 重构学生表
create table student (
id bigint PRIMARY KEY auto_increment, # 设置自增主键
name varchar(20) not null,
age int DEFAULT 18,
sno varchar(10) UNIQUE
);插入数据时不设置主键列的值
java
# 主键列的值为NULL
insert into student(id, name, sno) values (NULL, '张三', '100001');
Query OK, 1 row affected (0.00 sec)
# 不指定主键
insert into student(name, sno) values ('李四', '100002');
Query OK, 1 row affected (0.01 sec)
# 主键列的值自动生成
select * from student;
+----+--------+------+--------+
| id | name | age | sno |
+----+--------+------+--------+
| 1 | 张三 | 18 | 100001 |
| 2 | 李四 | 18 | 100002 |
+----+--------+------+--------+
2 rows in set (0.00 sec)查看表结构，Extra列显示auto_increment 表示自增
java
desc student;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | bigint | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | int | YES | | 18 | |
| sno | varchar(10) | YES | UNI | NULL | |
+-------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)如果某条记录写入失败，新生成的主键值将会作废
java
# 由于学号重复，产生了唯一冲突，导致插入失败，ID为3的主键值作废
insert into student(name, sno) values ('王五', '100002');
ERROR 1062 (23000): Duplicate entry '100002' for key 'student.sno'
# 修改学号后成功插入数据
insert into student(name, sno) values ('王五', '100003');
Query OK, 1 row affected (0.00 sec)
# 查询后发现新记录ID列的值为4
select * from student;
+----+--------+------+--------+
| id | name | age | sno |
+----+--------+------+--------+
| 1 | 张三 | 18 | 100001 |
| 2 | 王五 | 18 | 100002 |
| 4 | 王五 | 18 | 100003 | # 新插入的记录
+----+--------+------+--------+
3 rows in set (0.00 sec)主键值可以不连续
java
# 手动指定一个值
insert into student(id, name, sno) values (100, '赵六', '100004');
Query OK, 1 row affected (0.01 sec)
select * from student;
+-----+--------+------+--------+
| id | name | age | sno |
+-----+--------+------+--------+
| 1 | 张三 | 18 | 100001 |
| 2 | 王五 | 18 | 100002 |
| 4 | 王五 | 18 | 100003 |
| 100 | 赵六 | 18 | 100004 |
+-----+--------+------+--------+
4 rows in set (0.00 sec)
# 下一次自增从主键的最大值开始
insert into student(name, sno) values ('钱七', '100005');
Query OK, 1 row affected (0.01 sec)
select * from student;
+-----+--------+------+--------+
| id | name | age | sno |
+-----+--------+------+--------+
| 1 | 张三 | 18 | 100001 |
| 2 | 王五 | 18 | 100002 |
| 4 | 王五 | 18 | 100003 |
| 100 | 赵六 | 18 | 100004 |
| 101 | 钱七 | 18 | 100005 | # ID列的值是101
+-----+--------+------+--------+
5 rows in set (0.00 sec)主键或唯一键冲突时的更新操作，插入否则更新
java
# 语法
INSERT ... ON DUPLICATE KEY UPDATE column = value [, column = value] ...
# 插入ID为100，学号为100100的学生记录时，报主键冲突
insert into student(id, name, sno) values (100, '赵六', '100100');
ERROR 1062 (23000): Duplicate entry '100' for key 'student.PRIMARY'
# 可以使用以上语法，如果插入时有冲突则更新当前列的值
insert into student(id, name, sno) values (100, '赵六', '100100')
-> ON DUPLICATE KEY UPDATE name = '赵六', sno = '100100';
Query OK, 2 rows affected (0.01 sec)
# 两行受影响，表示删除了原来的记录，又新写入了一条记录
# 与update student set name = '赵六', sno = '100100' where id = 100; 等效
select * from student;
+-----+--------+------+--------+
| id | name | age | sno |
+-----+--------+------+--------+
| 1 | 张三 | 18 | 100001 |
| 2 | 王五 | 18 | 100002 |
| 4 | 王五 | 18 | 100003 |
| 100 | 赵六 | 18 | 100100 | # 学号已修改
| 101 | 钱七 | 18 | 100005 |
+-----+--------+------+--------+
5 rows in set (0.00 sec)替换，如果存在冲突则替换，不存在冲突则插入
java
# 语法
REPLACE [INTO] table_name
[(column [, column] ...)]
VALUES
(value_list) [, (value_list)] ...
value_list: value, [, value] ...
# 写入或更新Id为101的记录
REPLACE into student(id, name, sno) values (101, '钱七', '100101');
Query OK, 2 rows affected (0.01 sec) # 受影响两行
# 原数据已更新
select * from student;
+-----+--------+------+--------+
| id | name | age | sno |
+-----+--------+------+--------+
| 1 | 张三 | 18 | 100001 |
| 2 | 王五 | 18 | 100002 |
| 4 | 王五 | 18 | 100003 |
| 100 | 赵六 | 18 | 100100 |
| 101 | 钱七 | 18 | 100101 |
+-----+--------+------+--------+
5 rows in set (0.00 sec)
# 写入一条新数据
REPLACE into student(id, name, sno) values (102, '吴八', '100102');
Query OK, 1 row affected (0.01 sec) # 受影响一行
# 成功写入
select * from student;
+-----+--------+------+--------+
| id | name | age | sno |
+-----+--------+------+--------+
| 1 | 张三 | 18 | 100001 |
| 2 | 王五 | 18 | 100002 |
| 4 | 王五 | 18 | 100003 |
| 100 | 赵六 | 18 | 100100 |
| 101 | 钱七 | 18 | 100101 |
| 102 | 吴八 | 18 | 100102 |
+-----+--------+------+--------+
6 rows in set (0.00 sec)表中不能有多个主键
java
drop table student;
# 重构学生表
create table student (
id bigint PRIMARY KEY auto_increment, # 定义主键
name varchar(20) PRIMARY KEY # 定义主键
);
ERROR 1068 (42000): Multiple primary key defined # 报错复合主键：由多个列共同组成的主键，主键是否冲突以多个列的组成进行判定
java
drop table student;
# 重构学生表
create table student (
id bigint,
name varchar(20),
PRIMARY KEY (id, name) # 指定复合主键
);
# 插入数据
insert into student(id, name) values (1, '张三');
Query OK, 1 row affected (0.01 sec)
# 重复插入主键冲突，此时主键值由id和name两个列共同决定
insert into student(id, name) values (1, '张三');
ERROR 1062 (23000): Duplicate entry '1-张三' for key 'student.PRIMARY'
# 修改id值插入成功
insert into student(id, name) values (2, '张三');
Query OK, 1 row affected (0.00 sec)
select * from student;
+----+--------+
| id | name |
+----+--------+
| 1 | 张三 |
| 2 | 张三 |
+----+--------+
2 rows in set (0.00 sec)8. FOREIGN KEY 外键约束
java
drop table if exists class;
# 建表
create table class (
id bigint primary key auto_increment,
name varchar(20) not null
);
# 初始化数据
insert into class (name) values ('java01'), ('java02'), ('java03'), ('C++01'),
('C++02');
Records: 5 Duplicates: 0 Warnings: 0
select * from class;
+----+--------+
| id | name |
+----+--------+
| 1 | java01 |
| 2 | java02 |
| 3 | java03 |
| 4 | C++01 |
| 5 | C++02 |
+----+--------+
5 rows in set (0.00 sec)重构学生表(从表)，加入外键约束
java
drop table if exists student;
# 重构表
create table student(
id bigint PRIMARY KEY auto_increment,
name varchar(20) not null,
age int DEFAULT 18,
class_id bigint,
foreign key (class_id) references class(id) # 创建外键约束
);查看表结构，Key列的值为MUL表示外键约束的列
java
desc student;
+----------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+----------------+
| id | bigint | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| age | int | YES | | 18 | |
| class_id | bigint | YES | MUL | NULL | |
+----------+-------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)正常插入数据
java
# 班级编号在主表中存在
insert into student(name, class_id) values ('张三', 1), ('李四', 2);
Query OK, 2 rows affected (0.01 sec)
Records: 2 Duplicates: 0 Warnings: 0
# 插入成功
select * from student;
+----+--------+------+----------+
| id | name | age | class_id |
+----+--------+------+----------+
| 1 | 张三 | 18 | 1 |
| 2 | 李四 | 18 | 2 |
+----+--------+------+----------+
2 rows in set (0.00 sec)插入一个班级号为100的学生，由于主表中没有这个班级，插入失败
java
# 班级编号在主表中不存在，提示外键约束限制导致插入失败
insert into student(name, class_id) values ('王五', 100);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint
fails (`java01`.`student`, CONSTRAINT `student_ibfk_1` FOREIGN KEY
(`class_id`) REFERENCES `class` (`id`))插入班级Id为NULL的记录，可以成功，表示当前学生还没有分配置班级
java
# 班级为NULL
insert into student(name, class_id) values ('王五', NULL);
Query OK, 1 row affected (0.00 sec)
select * from student;
+----+--------+------+----------+
| id | name | age | class_id |
+----+--------+------+----------+
| 1 | 张三 | 18 | 1 |
| 2 | 李四 | 18 | 2 |
| 4 | 王五 | 18 | NULL |
+----+--------+------+----------+
3 rows in set (0.00 sec)删除主表某条记录时，从表中不能有对该记录的引用
java
# 删除从表中没有引用的记录，可以成功
delete from class where name = 'java03';
Query OK, 1 row affected (0.00 sec)
select * from class;
+----+--------+
| id | name |
+----+--------+
| 1 | java01 |
| 2 | java02 |
| 4 | C++01 |
| 5 | C++02 |
+----+--------+
4 rows in set (0.00 sec)
# 删除从表中引用的记录，失败
delete from class where name = 'java01';
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key
constraint fails (`java01`.`student`, CONSTRAINT `student_ibfk_1` FOREIGN KEY
(`class_id`) REFERENCES `class` (`id`))删除主表时要先删除从表
java
# 从表存在是不能删除主表
drop table class;
ERROR 3730 (HY000): Cannot drop table 'class' referenced by a foreign key
constraint 'student_ibfk_1' on table 'student'.
# 删除从表
drop table student;
Query OK, 0 rows affected (0.02 sec)
# 再删除主表，成功
drop table class;
Query OK, 0 rows affected (0.01 sec)9. CHECK 约束
重构学生表，有以下要求，年龄不能小于16岁，性别只能是男或女
java
drop table if exists student;
# 加入CHECK约束
create table student(
id bigint PRIMARY KEY auto_increment, # 设置自增主键
name varchar(20) not null,
age int DEFAULT 18,
gender char(1),
check (age >= 16),
check (gender = '男' or gender = '女')
);
# 正常插入数据
mysql> insert into student(name, age, gender) values ('张三', 17, '男'), ('李
四', 19, '女');
Query OK, 2 rows affected (0.01 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from student;
+----+--------+------+--------+
| id | name | age | gender |
+----+--------+------+--------+
| 1 | 张三 | 17 | 男 |
| 2 | 李四 | 19 | 女 |
+----+--------+------+--------+
2 rows in set (0.00 sec)
# 插入年龄小于16岁的记录，失败
mysql> insert into student(name, age, gender) values ('张三', 15, '男');
ERROR 3819 (HY000): Check constraint 'student_chk_1' is violated.
# 插入性别的值不是男或女的记录，失败
mysql> insert into student(name, age, gender) values ('张三', 17, '1');
ERROR 3819 (HY000): Check constraint 'student_chk_2' is violated创建新表，c1的值不能为0，c2的值必须大于0，c3的值不小于c2
java
# 列与列之间也可以比较，需要在单独一行中定义
create table t_check (
c1 int check(c1 <> 0),
c2 int check(c2 > 0),
c3 int,
check(c3 >= c2)
);
# 插入正常数据
mysql> insert into t_check values (-1, 3, 10);
Query OK, 1 row affected (0.01 sec)
mysql> select * from t_check;
+------+------+------+
| c1 | c2 | c3 |
+------+------+------+
| -1 | 3 | 10 |
+------+------+------+
1 row in set (0.00 sec)
# c1 = 0时，失败
mysql> insert into t_check values (0, 5, 6);
ERROR 3819 (HY000): Check constraint 't_check_chk_1' is violated.
# c2 <= 0时，失败
mysql> insert into t_check values (2, -10, 10);
ERROR 3819 (HY000): Check constraint 't_check_chk_2' is violated.
# c3 < c2时，失败
mysql> insert into t_check values (2, 10, 9);
ERROR 3819 (HY000): Check constraint 't_check_chk_3' is violated.