oracle 表添加主键 primary key

根据数据库三范式就是每个表建议要有主键，每个字段不可再分，表与之间不存在部分依赖
数据库的优化首先考虑的也是主键
主键会在对应的字段加唯一约束、加not null,建 索引
怎么为表加主键呢
1，建表同时
SQL> create table test13 (id int primary key);
Table created.
2，建表后加字段时
SQL> alter table test12 add id2 int primary key;
Table altered.
3,主键字段存在时
SQL> select constraint_name from user_constraints where table_name='TEST12';
CONSTRAINT_NAME
------------------------------------------------------------
SYS_C0010610
SQL> alter table test12 drop constraint SYS_C0010610
2  ;
Table altered.
SQL> alter table test12 add id1 int;
Table altered.
SQL> alter table test12 add constraint pk_id1_t12  primary key(id1);
Table altered.
完成！
ps:如果在要加主键的字段的值中有空值或不唯一，就会报错，有空值时报
SQL> alter table test12 add constraint pk_id1_t12  primary key(id1);
alter table test12 add constraint pk_id1_t12  primary key(id1)
*
ERROR at line 1:
ORA-01449: column contains NULL values; cannot alter to NOT NULL