在Oracle数据库中，关于约束的状态有下面两个参数：
initially （initially immediate 或 initially deferred）
deferrable（deferrable 或 not deferrable）
第1个参数，指定默认情况下，约束的验证时刻（在事务每条子句结束时，还是在整个事务结束时）。
第2个参数，指定了在事务中，是否可以改变上一条参数的设置。
如果不指定上述参数，默认设置是 initially immediate not deferrable。
注意：如果约束是not deferrable，那么它只能是initially immediate，而不能是initially deferred。
测试①，initially immediate：
SQL> create table nlist (
2 nid number
3 );
Table created
SQL> alter table nlist add constraint pk_nlist primary key (nid) initially immediate;
Table altered
SQL> insert into nlist values (1);
1 row inserted
SQL> insert into nlist values (1);
insert into nlist values (1)
ORA-00001: 违反唯一约束条件 (TEST.PK_NLIST)
测试②，initially deferred：
SQL> create table nlist (
2 nid number
3 );
Table created
SQL> alter table nlist add constraint pk_nlist primary key (nid) initially deferred;
Table altered
SQL> insert into nlist values (1);
1 row inserted
SQL> insert into nlist values (1);
1 row inserted
SQL> commit;
commit
ORA-02091: 事务处理已回退
ORA-00001: 违反唯一约束条件 (TEST.PK_NLIST)
测试③，initially immediate deferrable：
SQL> create table nlist (
2 nid number
3 );
Table created
SQL> alter table nlist add constraint pk_nlist primary key (nid) initially immediate deferrable;
Table altered
SQL> set constraint pk_nlist deferred;
Constraints set
SQL> insert into nlist values (1);
1 row inserted
SQL> insert into nlist values (1);
1 row inserted
SQL> commit;
commit
ORA-02091: 事务处理已回退
ORA-00001: 违反唯一约束条件 (TEST.PK_NLIST)
测试④，initially deferred deferrable：
SQL> create table nlist (
2 nid number
3 );
Table created
SQL> alter table nlist add constraint pk_nlist primary key (nid) initially deferred deferrable;
Table altered
SQL> set constraint pk_nlist immediate;
Constraints set
SQL> insert into nlist values (1);
1 row inserted
SQL> insert into nlist values (1);
insert into nlist values (1)
ORA-00001: 违反唯一约束条件 (TEST.PK_NLIST)
测试⑤：
SQL> create table nlist (
2 nid number
3 );
Table created
SQL> alter table nlist add constraint pk_nlist primary key (nid) initially deferred no deferrable;
alter table nlist add constraint pk_nlist primary key (nid) initially deferred no deferrable
ORA-01735: 无效的 ALTER TABLE 选项
