一、准备知识
约束放置在表中，有以下五种约束:
NOT NULL 非空约束C 指定的列不允许为空值
UNIQUE 唯一约束U 指定的列中没有重复值，或该表中每一个值或者每一组值都将是唯一的
PRIMARY KEY 主键约束P 唯一的标识出表的每一行,且不允许空值值,一个表只能有一个主键约束
FOREIGN KEY 外键约束R 一个表中的列引用了其它表中的列，使得存在依赖关系，可以指向引用自身的列
CHECK 条件约束C 指定该列是否满足某个条件
约束命名规则
如果不指定约束名Oracle server 自动按照SYS_Cn 的格式指定约束名，也可手动指定，
推荐的约束命名是：约束类型_表名_列名。
NN：NOT NULL 非空约束，比如nn_emp_sal
UK：UNIQUE KEY 唯一约束
PK：PRIMARY KEY 主键约束
FK：FOREIGN KEY 外键约束
CK：CHECK 条件约束
外键约束是用来维护从表和主表的引用完整性的，所以外键约束要涉及两个表。
FOREIGN KEY: 在表级指定子表中的列
REFERENCES: 标示在父表中的列
ON DELETE CASCADE: 当父表中的列被删除时，子表中相对应的列也被删除
ON DELETE SET NULL: 子表中相应的列置空
二、外键创建测试
foreign_main为主表
foreign_sub为从表
object_id做为foreign_sub的外键，参考主表foreign_main的object_id值
SQL> create table foreign_main as select object_id from all_objects;
Table created.
SQL> select count(*) from foreign_main;
COUNT(*)
----------
49571
SQL> create table foreign_sub as select object_id,object_name from all_objects;
Table created.
建议使用主表的主键做外键，即使不是主表的主键也应该是唯一约束的字段做为外键
SQL> alter table foreign_main add constraint pk_fsid primary key(object_id);
Table altered.
SQL> delete from foreign_sub where object_name = 'FOREIGN_MAIN';
1 row deleted.
SQL> commit;
Commit complete.
SQL> alter table foreign_sub add constraint fr_fssid foreign key(object_id) references foreign_main(object_id);
Table altered.
从表插入一条主表object_id中不存在的记录测试
SQL> insert into foreign_sub values(1,'ts');
insert into foreign_sub values(1,'ts')
*
ERROR at line 1:
ORA-02291: integrity constraint (TEST.FR_FSSID) violated - parent key not found
提示主表数据不存在，从表不能创建主表不存在的object_id以保证完整性
三、级联删除测试
SQL> alter table foreign_sub drop constraint fk_fs_oid;
Table altered.
SQL> alter table foreign_sub add constraint fk_fs_oid foreign key(object_id) references foreign_main(object_id) on delete cascade;
Table altered.
cascade下仍然不能单独更新主表外键字段
SQL> update foreign_main set object_id=52012 where object_id=52010;
update foreign_main set object_id=52012 where object_id=52010
*
ERROR at line 1:
ORA-02292: integrity constraint (TEST.FK_FS_OID) violated - child record found
cascade模式下可以通过主表删除外键字段数据关联删除从表数据
SQL> select * from foreign_sub where object_id=52010;
OBJECT_ID OBJECT_NAME
---------- ------------------------------
52010 IDX_BJNAME
SQL> delete from foreign_main where object_id=52010;
1 row deleted.
SQL> commit;
Commit complete.
SQL> select * from foreign_sub where object_id=52010;
no rows selected
外键相关常用操作及参考文档
建立外键
alter table 表名 add constraint 外键名 foreign key(从表外键字段) references foreign_main(主表外键字段);
drop表外键
alter table 表名 drop constraint 外键名;
通过外键找表
select * from user_constraints where constraint_type='R' and constraint_name=upper('外键名');
通过表找外键
select * from user_constraints where constraint_type='R' and table_name=upper('表名');
查找表的外键(包括名称，引用表的表名和对应的键名，下面是分成多步查询)：
select * from user_constraints c where c.constraint_type = 'R' and c.table_name = 要查询的表
查询引用表的键的列名：
select * from user_cons_columns cl where cl.constraint_name = 外键引用表的键名
外键约束临时disabled
alter table 表名 disable constraint 外键名;
在SQL92标准中定义了几种外键改变后，如何处理子表记录的动作，其中包括：
限制Restrict：这种方式不允许对被参考的记录的键值执行更新或删除的操作;置为空Set to null：当参考的数据被更新或者删除，那么所有参考它的外键值被置为空;
置为默认值Set to default：当参考的数据被更新或者删除，那么所有参考它的外键值被置为一个默认值;
级联Cascade：当参考的数据被更新，则参考它的值同样被更新，当参考的数据被删除，则参考它的子表记录也被删除;
不做操作No action：这种方式不允许更新或删除被参考的数据。和限制方式的区别在于，这种方式的检查发生在语句执行之后。Oracle默认才会的方式就是这种方式。
Col OWNER FOR A6
COL R_OWNER FOR A6
COL TABLE_NAME FOR A15
select OWNER, TABLE_NAME, CONSTRAINT_NAME, CONSTRAINT_TYPE, R_OWNER, R_CONSTRAINT_NAME, DELETE_RULE from user_constraints where table_name in ('FOREIGN_MAIN', 'FOREIGN_SUB');
Select CONSTRAINT_NAME from user_constraints e where e.table_name='IMS_COLUMN' and owner='WSJD_ELMS6';
Select b.table_name,b.column_name, A.CONSTRAINT_TYPE, C.TABLE_NAME from user_constraints a, user_cons_columns b, user_constraints C
WHERE a.constraint_name = b.constraint_name AND
A.R_CONSTRAINT_NAME = C.CONSTRAINT_NAME
AND a.r_constraint_name IN (Select CONSTRAINT_NAME from user_constraints e where e.table_name='FOREIGN_MAIN' and owner='SCOTT');
create table foreign_sub as select object_id, object_name from user_objects;
create table foreign_main as select object_id from foreign_sub;
alter table foreign_main add constraint pk_foreign_main_object_id primary key(object_id);
alter table foreign_sub add constraint fr_foreign_sub_object_id foreign key(object_id) references foreign_main(object_id) on delete cascade;
alter table foreign_sub drop constraint fr_foreign_sub_object_id;
alter table foreign_sub disable constraint fr_foreign_sub_object_id;
如在goldengate 没有禁用外键约束会出现以现错误
=============================================
2013-12-26 04:51:25 INFO OGG-00996 Oracle GoldenGate Delivery for Oracle, rep_app.prm: REPLICAT REP_APP started.
2013-12-26 04:51:25 WARNING OGG-00869 Oracle GoldenGate Delivery for Oracle, rep_app.prm: OCI Error ORA-02292: integrity constraint (SCOTT.FR_FOREIGN_SUB_OBJECT_ID) violated - child record found (status = 2292). DELETE FROM "SCOTT"."FOREIGN_MAIN" WHERE "OBJECT_ID" = :b0.
2013-12-26 04:51:25 WARNING OGG-01004 Oracle GoldenGate Delivery for Oracle, rep_app.prm: Aborted grouped transaction on 'SCOTT.FOREIGN_MAIN', Database error 2292 (OCI Error ORA-02292: integrity constraint (SCOTT.FR_FOREIGN_SUB_OBJECT_ID) violated - child record found (status = 2292). DELETE FROM "SCOTT"."FOREIGN_MAIN" WHERE "OBJECT_ID" = :b0).
2013-12-26 04:51:25 WARNING OGG-01003 Oracle GoldenGate Delivery for Oracle, rep_app.prm: Repositioning to rba 17426 in seqno 29.
2013-12-26 04:51:25 WARNING OGG-01154 Oracle GoldenGate Delivery for Oracle, rep_app.prm: SQL error 2292 mapping SCOTT.FOREIGN_MAIN to SCOTT.FOREIGN_MAIN OCI Error ORA-02292: integrity constraint (SCOTT.FR_FOREIGN_SUB_OBJECT_ID) violated - child record found (status = 2292). DELETE FROM "SCOTT"."FOREIGN_MAIN" WHERE "OBJECT_ID" = :b0.
2013-12-26 04:51:25 WARNING OGG-01003 Oracle GoldenGate Delivery for Oracle, rep_app.prm: Repositioning to rba 17426 in seqno 29.
2013-12-26 04:51:25 ERROR OGG-01296 Oracle GoldenGate Delivery for Oracle, rep_app.prm: Error mapping from SCOTT.FOREIGN_MAIN to SCOTT.FOREIGN_MAIN.
2013-12-26 04:51:25 WARNING OGG-01525 Oracle GoldenGate Delivery for Oracle, rep_app.prm: Failed to open trace output file, 'gglog-REP_APP.dmp', error 13 (Permission denied).
2013-12-26 04:51:25 ERROR OGG-01668 Oracle GoldenGate Delivery for Oracle, rep_app.prm: PROCESS ABENDING.
2013-12-26 04:52:20 INFO OGG-00996 Oracle GoldenGate Delivery for Oracle, rep_app.prm: REPLICAT REP_APP started.
2013-12-26 04:52:20 WARNING OGG-00869 Oracle GoldenGate Delivery for Oracle, rep_app.prm: No unique key is defined for table 'FOREIGN_SUB'. All viable columns will be used to represent the key, but may not guarantee uniqueness. KEYCOLS may be used to define the key.
2013-12-26 04:52:20 WARNING OGG-01004 Oracle GoldenGate Delivery for Oracle, rep_app.prm: Aborted grouped transaction on 'SCOTT.FOREIGN_SUB', Database error 1403 (OCI Error ORA-01403: no data found, SQL ).
2013-12-26 04:52:20 WARNING OGG-01003 Oracle GoldenGate Delivery for Oracle, rep_app.prm: Repositioning to rba 17426 in seqno 29.
2013-12-26 04:52:20 WARNING OGG-01154 Oracle GoldenGate Delivery for Oracle, rep_app.prm: SQL error 1403 mapping SCOTT.FOREIGN_SUB to SCOTT.FOREIGN_SUB OCI Error ORA-01403: no data found, SQL .
2013-12-26 04:52:20 WARNING OGG-01003 Oracle GoldenGate Delivery for Oracle, rep_app.prm: Repositioning to rba 17426 in seqno 29.
2013-12-26 04:52:20 ERROR OGG-01296 Oracle GoldenGate Delivery for Oracle, rep_app.prm: Error mapping from SCOTT.FOREIGN_SUB to SCOTT.FOREIGN_SUB.
2013-12-26 04:52:20 WARNING OGG-01525 Oracle GoldenGate Delivery for Oracle, rep_app.prm: Failed to open trace output file, 'gglog-REP_APP.dmp', error 13 (Permission denied).
2013-12-26 04:52:20 ERROR OGG-01668 Oracle GoldenGate Delivery for Oracle, rep_app.prm: PROCESS ABENDING.