约束的4种状态：
disable novalidate 既不会约束新增数据也不会验证已有数据，等同于disable
enable novalidate 约束新增数据但不会验证已有数据
disable validate 约束新增数据但不会验证已有数据，启用后禁止DML
enable validate 约束新增数据并验证已有数据，等同于enable
扩展解释：
disable / enable 针对新增数据（对于唯一性约束，enable会自动创建唯一性索引，disable会自动drop索引）；
novalidate / validate 针对已有数据。
测试：
Connected to Oracle Database 11g Enterprise Edition Release 11.2.0.1.0
Connected as system
SQL> drop table dept2;
Table dropped
SQL> create table dept2 as select * from scott.dept;
Table created
SQL> select * from dept2 order by deptno;
DEPTNO DNAME LOC
------ -------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SQL> alter table dept2 add constraint dept2_u1 unique(deptno);
Table altered
SQL> select index_name,table_name,uniqueness from dba_indexes where index_name = 'DEPT2_U1';
INDEX_NAME TABLE_NAME UNIQUENESS
------------------------------ ------------------------------ ----------
DEPT2_U1 DEPT2 UNIQUE
SQL> select constraint_name,status,validated from dba_constraints where constraint_name = 'DEPT2_U1';
CONSTRAINT_NAME STATUS VALIDATED
------------------------------ -------- -------------
DEPT2_U1 ENABLED VALIDATED
SQL> insert into dept2(deptno) values(10);
insert into dept2(deptno) values(10)
ORA-00001: unique constraint (SYSTEM.DEPT2_U1) violated
SQL> select * from dept2 order by deptno;
DEPTNO DNAME LOC
------ -------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SQL> alter table dept2 modify constraint dept2_u1 disable novalidate;
Table altered
SQL> select index_name,table_name,uniqueness from dba_indexes where index_name = 'DEPT2_U1';
INDEX_NAME TABLE_NAME UNIQUENESS
------------------------------ ------------------------------ ----------
（disable自动移除索引）
SQL> select constraint_name,status,validated from dba_constraints where constraint_name = 'DEPT2_U1';
CONSTRAINT_NAME STATUS VALIDATED
------------------------------ -------- -------------
DEPT2_U1 DISABLED NOT VALIDATED
SQL> insert into dept2(deptno) values(10);
1 row inserted
SQL> select * from dept2 order by deptno;
DEPTNO DNAME LOC
------ -------------- -------------
10 ACCOUNTING NEW YORK
10
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SQL> alter table dept2 modify constraint dept2_u1 enable novalidate;
alter table dept2 modify constraint dept2_u1 enable novalidate
ORA-02299: cannot validate (SYSTEM.DEPT2_U1) - duplicate keys found
（因为enable会去创建唯一性索引，而已有数据deptno存在重复数据10，所以这里不能enable）
SQL> delete from dept2 where deptno=10 and dname is null;
1 row deleted
SQL> alter table dept2 modify constraint dept2_u1 enable novalidate;
Table altered
SQL> select index_name,table_name,uniqueness from dba_indexes where index_name = 'DEPT2_U1';
INDEX_NAME TABLE_NAME UNIQUENESS
------------------------------ ------------------------------ ----------
DEPT2_U1 DEPT2 UNIQUE
（enable会自动创建唯一性索引）
SQL> select constraint_name,status,validated from dba_constraints where constraint_name = 'DEPT2_U1';
CONSTRAINT_NAME STATUS VALIDATED
------------------------------ -------- -------------
DEPT2_U1 ENABLED NOT VALIDATED
SQL> insert into dept2(deptno) values(10);
insert into dept2(deptno) values(10)
ORA-00001: unique constraint (SYSTEM.DEPT2_U1) violated
SQL> alter table dept2 modify constraint dept2_u1 disable validate;
Table altered
SQL> select index_name,table_name,uniqueness from dba_indexes where index_name = 'DEPT2_U1';
INDEX_NAME TABLE_NAME UNIQUENESS
------------------------------ ------------------------------ ----------
SQL> select constraint_name,status,validated from dba_constraints where constraint_name = 'DEPT2_U1';
CONSTRAINT_NAME STATUS VALIDATED
------------------------------ -------- -------------
DEPT2_U1 DISABLED VALIDATED
SQL> insert into dept2(deptno) values(10);
insert into dept2(deptno) values(10)
ORA-25128: No insert/update/delete on table with constraint (SYSTEM.DEPT2_U1) disabled and validated
（disable validate后禁止DML）
SQL> select * from dept2 order by deptno;
DEPTNO DNAME LOC
------ -------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SQL> alter table dept2 modify constraint dept2_u1 enable validate;
Table altered
SQL> select index_name,table_name,uniqueness from dba_indexes where index_name = 'DEPT2_U1';
INDEX_NAME TABLE_NAME UNIQUENESS
------------------------------ ------------------------------ ----------
DEPT2_U1 DEPT2 UNIQUE
SQL> select constraint_name,status,validated from dba_constraints where constraint_name = 'DEPT2_U1';
CONSTRAINT_NAME STATUS VALIDATED
------------------------------ -------- -------------
DEPT2_U1 ENABLED VALIDATED
SQL> insert into dept2(deptno) values(10);
insert into dept2(deptno) values(10)
ORA-00001: unique constraint (SYSTEM.DEPT2_U1) violated
SQL> select * from dept2 order by deptno;
DEPTNO DNAME LOC
------ -------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SQL>
REF: <<Expert Oracle Database 11g Administration>> Page 308, Integrity Constraint States
