Oracle中Constraint的状态参数initially与deferrable

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
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