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一、Oracle约束的状态
Oracle完整性约束的状态有4种，分别是ENABLE、DISABLE、VALIDATE、NOVALIDATE。
ENABLE 表示Oracle将检查要插入或更新的数据库中的数据是否符合约束；
DISABLE 表示表中可以存放违反约束的行；
VALIDATE 表示数据库验证表中的已存在数据是否符合约束；
NOVALIDATE 表示数据库不验证表中已存在数据是否符合约束。
Oracle默认约束状态为ENABLE、VALIDATE。
下面看Oracle官方给出的汇总：
| Modified Data | Existing Data | Summary |
|---|---|---|
|
| Existing and future data must obey the constraint. An attempt to apply a new constraint to a populated table results in an error if existing rows violate the constraint. |
|
| The database checks the constraint, but it need not be true for all rows. Thus, existing rows can violate the constraint, but new or modified rows must conform to the rules. |
|
| The database disables the constraint, drops its index, and prevents modification of the constrained columns. |
|
| The constraint is not checked and is not necessarily true. |
下面使用实例测试各状态：
创建测试表
zx@ORA11G>create table t1 (id number,name varchar2(10),address varchar2(10)); Table created. zx@ORA11G>insert into t1 values(1,'zx','hb'); 1 row created. zx@ORA11G>insert into t1 values(1,'zq','jx'); 1 row created. zx@ORA11G>insert into t1 values(2,'wl','sd'); 1 row created. zx@ORA11G>commit; Commit complete.1、测试ENABLE、VALIDATE状态
zx@ORA11G>alter table t1 add constraint t1_uk unique(id); alter table t1 add constraint t1_uk unique(id) * ERROR at line 1: ORA-02299: cannot validate (ZX.T1_UK) - duplicate keys found因为id列中有重复值，此时创建约束t1_uk的状态为ENABLE、VALIDATE会验证表中已存在的数据，所以创建约束不成功。删除表中的重复数据再次创建约束即可成功。
zx@ORA11G>delete from t1 where id=1 and name='zq'; 1 row deleted. zx@ORA11G>commit; Commit complete. zx@ORA11G>alter table t1 add constraint t1_uk unique(id); Table altered. zx@ORA11G>select table_name,constraint_name,constraint_type,deferrable,status,validated from user_constraints where table_name='T1'; TABLE_NAME CONSTRAINT_NAME C DEFERRABLE STATUS VALIDATED ------------------------------ ------------------------------ - -------------- -------- ------------- T1 T1_UK U NOT DEFERRABLE ENABLED VALIDATED创建完成后再次插入id=1的数据即会报错，说明约束状态为ENABLE
zx@ORA11G>insert into t1 values(1,'zq','jx'); insert into t1 values(1,'zq','jx') * ERROR at line 1: ORA-00001: unique constraint (ZX.T1_UK) violated2、测试ENABLE、DISABLED状态
zx@ORA11G>select * from t1; ID NAME ADDRESS ---------- ---------- ---------- 1 zx hb 2 wl sd 1 zq jx zx@ORA11G>alter table t1 add constraint t1_uk unique(id) enable novalidate; alter table t1 add constraint t1_uk unique(id) enable novalidate * ERROR at line 1: ORA-02299: cannot validate (ZX.T1_UK) - duplicate keys found直接创建unique约束报错，因为有重复值。但先在id列上创建索引，然后创建unique约束即可成功。
zx@ORA11G>create index idx_t_id on t1(id); Index created. zx@ORA11G>alter table t1 add constraint t1_uk unique(id) using index idx_t_id enable novalidate; Table altered. zx@ORA11G>select * from t1; ID NAME ADDRESS ---------- ---------- ---------- 1 zx hb 2 wl sd 1 zq jx zx@ORA11G>select table_name,constraint_name,constraint_type,deferrable,status,validated from user_constraints where table_name='T1'; TABLE_NAME CONSTRAINT_NAME C DEFERRABLE STATUS VALIDATED ------------------------------ ------------------------------ - -------------- -------- ------------- T1 T1_UK U NOT DEFERRABLE ENABLED NOT VALIDATED原表中的id列中有重复值，还是可以创建unique约束，因为状态指定为NOVALIDATE，不验证表中已有的数据。另外因为状态为ENABLE，再次插入重复值报错：
zx@ORA11G>insert into t1 values(2,'yc','bj'); insert into t1 values(2,'yc','bj') * ERROR at line 1: ORA-00001: unique constraint (ZX.T1_UK) violated3、测试DISABLE、VALIDATE状态
zx@ORA11G>select * from t1; ID NAME ADDRESS ---------- ---------- ---------- 1 zx hb 2 wl sd zx@ORA11G>alter table t1 add constraint t1_uk unique(id) using index idx_t_id disable validate; Table altered. zx@ORA11G>select table_name,constraint_name,constraint_type,deferrable,status,validated from user_constraints where table_name='T1'; TABLE_NAME CONSTRAINT_NAME C DEFERRABLE STATUS VALIDATED ------------------------------ ------------------------------ - -------------- -------- ------------- T1 T1_UK U NOT DEFERRABLE DISABLED VALIDATED zx@ORA11G>insert into t1 values(1,'zq','jx'); insert into t1 values(1,'zq','jx') * ERROR at line 1: ORA-25128: No insert/update/delete on table with constraint (ZX.T1_UK) disabled and validatedDISABLE、VALIDATE状态下，不允许做增删改操作。
4、测试DISABLE、NOVALIDATE状态
zx@ORA11G>select * from t1; ID NAME ADDRESS ---------- ---------- ---------- 1 zx hb 2 wl sd 1 zq jx zx@ORA11G>alter table t1 add constraint t1_uk unique(id) using index idx_t_id disable novalidate; Table altered. zx@ORA11G>select table_name,constraint_name,constraint_type,deferrable,status,validated from user_constraints where table_name='T1'; TABLE_NAME CONSTRAINT_NAME C DEFERRABLE STATUS VALIDATED ------------------------------ ------------------------------ - -------------- -------- ------------- T1 T1_UK U NOT DEFERRABLE DISABLED NOT VALIDATED zx@ORA11G>insert into t1 values(2,'yc','bj'); 1 row created. zx@ORA11G>commit; Commit complete. zx@ORA11G>select * from t1; ID NAME ADDRESS ---------- ---------- ---------- 1 zx hb 2 wl sd 1 zq jx 2 yc bj约束状态为DISABLE、NOVALIDATE，对新数据和老数据都不做验证。
二、验证机制
1. 两种验证时机.
Oracle的constraints(约束) 根据验证时机可以分成两种.
case 1. 在每一句insert statement 执行时就会马上验证, 如果约束验证失败, 则这句sql statement 会执行失败.
case 2. 执行insert statements 时不会验证, 在commit的时候验证, 如果验证失败, 则整个Transaction 回滚.
2.constraints的分类
对应地, oracle 的 constraints 也可以分成两大类.
一种是not deferrable (不可以延时的) . 这种情况下只能执行 case1 的验证时机(即时验证)
另一种是 deferrable (可以设置成延时的). 这种情况下可以执行 case 1 或 case2 的验证时机. 但需要设置.
对于第二种defferable 分类, 还可以分成两小类.
一种是 initially immediate , 意思时默认情况下执行case 1.
另一种是initially deferred, 意思是默认情况下执行case2.
也就是可以分成三种，如下图：
2.1、not deferrable
这种最常见也最简单. 如果在增加1个constraint 时不指定验证时机属性. 默认情况下就会被设为not deferrable.既然constraint 是不可以延时验证的, 所以也不用设定它的初始属性(实际上就是initially immediate)。
清空上面的t1表，并创建一个unique约束
zx@ORA11G>truncate table t1; Table truncated. zx@ORA11G>select * from t1; no rows selected zx@ORA11G>alter table t1 add constraint t1_uk unique (id) not deferrable; Table altered. zx@ORA11G>select table_name,constraint_name,constraint_type,deferrable,status,validated from user_constraints where table_name='T1'; TABLE_NAME CONSTRAINT_NAME C DEFERRABLE STATUS VALIDATED ------------------------------ ------------------------------ - -------------- -------- ------------- T1 T1_UK U NOT DEFERRABLE ENABLED VALIDATED约束为NOT DEFERRABLE状态，插入测试数据查看状态：
zx@ORA11G>insert into t1 values(1,'zx','hb'); 1 row created. zx@ORA11G>insert into t1 values(2,'wl','sd'); 1 row created. zx@ORA11G>insert into t1 values(1,'zq','jx'); insert into t1 values(1,'zq','jx') * ERROR at line 1: ORA-00001: unique constraint (ZX.T1_UK) violated zx@ORA11G>select * from t1; ID NAME ADDRESS ---------- ---------- ---------- 1 zx hb 2 wl sd插入第三条数据时因为有重复数据，直接报错，说明验证时机为case1:即时验证，但不会回滚之前插入的结果。
2.2、 deferrable、initially immediate状态
zx@ORA11G>alter table t1 drop constraint t1_uk; Table altered. zx@ORA11G>alter table t1 add constraint t1_uk unique (id) deferrable initially immediate; Table altered. zx@ORA11G>select table_name,constraint_name,constraint_type,deferrable,status,validated from user_constraints where table_name='T1'; TABLE_NAME CONSTRAINT_NAME C DEFERRABLE STATUS VALIDATED ------------------------------ ------------------------------ - -------------- -------- ------------- T1 T1_UK U DEFERRABLE ENABLED VALIDATED zx@ORA11G>insert into t1 values(1,'zx','hb'); 1 row created. zx@ORA11G>insert into t1 values(2,'wl','sd'); 1 row created. zx@ORA11G>insert into t1 values(1,'zq','jx'); insert into t1 values(1,'zq','jx') * ERROR at line 1: ORA-00001: unique constraint (ZX.T1_UK) violated zx@ORA11G>select * from t1; ID NAME ADDRESS ---------- ---------- ---------- 1 zx hb 2 wl sd插入第三条数据时报错因为有重复值，说明验证时机为case1:即时验证，这与前一种状态一样。那为什么还要设置这样一种状态呢？我们来执行下面的语句：
zx@ORA11G>set constraint t1_uk deferred; Constraint set.上面的语句并没有改变这个constraint的任何属性, 只不过是切换为另一种模式
也就是说初始是immediate模式的, 执行上面的语句后就临时变成deferred模式了.
再次执行前面的插入语句：
zx@ORA11G>insert into t1 values(1,'zx','hb'); 1 row created. zx@ORA11G>insert into t1 values(2,'wl','sd'); 1 row created. zx@ORA11G>insert into t1 values(1,'zq','jx'); 1 row created.第三条也能插入进去，下面尝试commit：
zx@ORA11G>commit; commit * ERROR at line 1: ORA-02091: transaction rolled back ORA-00001: unique constraint (ZX.T1_UK) violated zx@ORA11G>select * from t1; no rows selectedcommit时报错，查询t1表，没有任何数据，说明回滚了整个事务。即case2：延迟验证。此时再次执行上面的三次插入操作：
zx@ORA11G>insert into t1 values(1,'zx','hb'); 1 row created. zx@ORA11G>insert into t1 values(2,'wl','sd'); 1 row created. zx@ORA11G>insert into t1 values(1,'zq','jx'); insert into t1 values(1,'zq','jx') * ERROR at line 1: ORA-00001: unique constraint (ZX.T1_UK) violated从上面结果可以看出，插入第三行时又报错，说明上面的set constraint语句的作用范围只有当前的一个事务。事务结束后即约束状态即回到原模式。
2.3、deferrable、initially deferred
有了上面的第二个实验就可以很容易的理解这一状态了。
zx@ORA11G>alter table t1 drop constraint t1_uk; Table altered. zx@ORA11G>alter table t1 add constraint t1_uk unique (id) deferrable initially deferred; Table altered. zx@ORA11G>select table_name,constraint_name,constraint_type,deferrable,status,validated from user_constraints where table_name='T1'; TABLE_NAME CONSTRAINT_NAME C DEFERRABLE STATUS VALIDATED ------------------------------ ------------------------------ - -------------- -------- ------------- T1 T1_UK U DEFERRABLE ENABLED VALIDATED zx@ORA11G>insert into t1 values(1,'zx','hb'); 1 row created. zx@ORA11G>insert into t1 values(2,'wl','sd'); 1 row created. zx@ORA11G>insert into t1 values(1,'zq','jx'); 1 row created. zx@ORA11G>commit; commit * ERROR at line 1: ORA-02091: transaction rolled back ORA-00001: unique constraint (ZX.T1_UK) violated zx@ORA11G>select * from t1; no rows selected参考：http://blog.csdn.net/nvd11/article/details/12654691
http://docs.oracle.com/cd/E11882_01/server.112/e40540/datainte.htm#CNCPT33337
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