[ CONSTRAINT constraint_name ]
{ NOT NULL |
NULL |
CHECK ( expression ) [ NO INHERIT ] |
DEFAULT default_expr |
UNIQUE index_parameters |
PRIMARY KEY index_parameters |
REFERENCES reftable [ ( refcolumn ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
[ ON DELETE action ] [ ON UPDATE action ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

DEFERRABLE(可以推遲的約束檢查)：可以推遲到事務結尾。目前僅UNIQUE, PRIMARY KEY, EXCLUDE和FOREIGN KEY才支持DEFERRABLE。NOT NULL和CHECK約束不支持。

NOT DEFERRABLE(不可推遲的約束檢查)：在每一個命令后會立即驗證是否滿足約束條件。缺省是NOT DEFERRABLE。

如果約束是INITIALLY IMMEDIATE，那么每條語句之后就檢查它。這個是缺省。

如果約束是INITIALLY DEFERRED，那么直到事務完成才檢查。

檢查的時間可以用以下命令修改：

SET CONSTRAINTS [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

例子：

默認是NOT DEFERRABLE INITIALLY IMMEDIATE

create table t1 (id number, name char(10));

alter table t1 modify id number primary key;

SQL> insert into t1 values(1,'Tough1');
已創建 1 行。

SQL> insert into t1 values(1,'Tough1');
insert into t1 values(1,'Tough1')
*
第 1 行出現錯誤:
ORA-00001: 違反唯一約束條件 (SCOTT.SYS_C005428)

DEFERRABLE或INITIALLY DEFERRED的情況：

create table t2 (id number, name char(10));

alter table t2 modify id number primary key INITIALLY DEFERRED;

或

alter table t2 modify id number primary key DEFERRABLE INITIALLY DEFERRED;

SQL> insert into t2 values(1,'Tough1');

已創建 1 行。

SQL> insert into t2 values(1,'Tough1');

已創建 1 行。

SQL> commit;
commit
*
第 1 行出現錯誤:
ORA-02091: 事務處理已回退
ORA-00001: 違反唯一約束條件 (SCOTT.SYS_C005435)

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總結

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