MYSQL 的 primary key 和unique key 的区别

时间:2022-09-16 08:42:37

Primary key 与Unique Key都是唯一性约束。但二者有很大的区别:

1、Primary key的1个或多个列必须为NOT NULL,如果列为NULL,在增加PRIMARY KEY时,列自动更改为NOT NULL。而UNIQUE KEY 对列没有此要求。

2、一个表只能有一个PRIMARY KEY,但可以有多个UNIQUE KEY。

下面以测试说明:

SQL> create table t (a int,b int,c int,d int);

Table created.

SQL> desc t
Name                                      Null?    Type
----------------------------------------- -------- -----------

A                                                  NUMBER(38)
B                                                  NUMBER(38)
C                                                  NUMBER(38)
D                                                  NUMBER(38)

SQL> alter table t add constraint pk_t primary key (a,b);

Table altered.

SQL> desc t
Name                                      Null?    Type
----------------------------------------- -------- ----------------

A                                         NOT NULL NUMBER(38)
B                                         NOT NULL NUMBER(38)
C                                                  NUMBER(38)
D                                                  NUMBER(38)

可以看到A、B两个列都自动改为了NOT NULL

SQL> alter table t modify (a int null);
alter table t modify (a int null)
                      *
ERROR at line 1:
ORA-01451: column to be modified to NULL cannot be modified to NULL
可以看到,列A不允许改为NULL

SQL> alter table t drop constraint pk_t;

Table altered.

SQL> alter table t add constraint uk_t_1 unique (a,b);

Table altered.

SQL> desc t
Name                                      Null?    Type
----------------------------------------- -------- -----------

A                                                  NUMBER(38)
B                                                  NUMBER(38)
C                                                  NUMBER(38)
D                                                  NUMBER(38)

我们看到列A又变回了NULL。

注意到,在删除主键时,列的NULLABLE会回到原来的状态。如果在创建主键后,对原来为NULL的主键列,显式设为NOT NULL,在删除主键后仍然是NOT NULL。比如在创建主键后,执行下面的操作,可以看到:

SQL> alter table t modify (b int not null);

Table altered.

SQL> alter table t drop constraint pk_t;

Table altered.

SQL> desc t
Name                                      Null?    Type
----------------------------------------- -------- ----------

A                                                 NUMBER(38)
B                                         NOT NULL NUMBER(38)
C                                                  NUMBER(38)
D                                                  NUMBER(38)

再做如下的实验:

SQL> drop table t;

Table dropped.

SQL> create table t (a int,b int,c int,d int);

Table created.

SQL> alter table t add constraint uk_t_1 unique (a,b);

Table altered.

SQL> alter table t add constraint uk_t_2 unique (c,d);

Table altered.

可以看到可以增加两个UNIQUE KEY。看看能不能增加两个主键:

SQL> alter table t add constraint pk_t primary key (c);

Table altered.

SQL> alter table t add constraint pk1_t primary key (d);
alter table t add constraint pk1_t primary key (d)
                                  *
ERROR at line 1:
ORA-02260: table can have only one primary key
由此可以看到一个表只能有一个主键。

SQL> alter table t drop constraint pk_t;

Table altered.

SQL> insert into t (a ,b ) values (null,null);

1 row created.

SQL> /

1 row created.

SQL> insert into t (a ,b ) values (null,1);

1 row created.

SQL> /
insert into t (a ,b ) values (null,1)
*
ERROR at line 1:
ORA-00001: unique constraint (SYS.UK_T_1) violated


SQL> insert into t (a ,b ) values (1,null);

1 row created.

SQL> /
insert into t (a ,b ) values (1,null)
*
ERROR at line 1:
ORA-00001: unique constraint (SYS.UK_T_1) violated

主键和唯一键约束是通过参考索引实施的,如果插入的值均为NULL,则根据索引的原理,全NULL值不被记录在索引上,所以插入全NULL值时,可以有重复的,而其他的则不能插入重复值。