第三种解决方案:采用Oracle 可延迟约束Deferable。
约束可以是deferrable或not deferrable(默认)。
not deferrable 约束在每一个DML语句后检查;
deferrable 约束可以在每一个insert,delete,或update(即时模式)后立即检查,或者在事务末尾检查(延迟模式)
当没有按特定顺序执行数据加载时,这项功能特别有用——它允许先把数据载入子表,然后再装入父表。
另一种用法是在加载不符合某个check约束的数据之后,对其进行适当的更新。
语法如下:
[ [not] deferrable [initially {immediate | deferred} ] ]
或
[ [initially {immediate | deferred} ] [not] deferrable ]
- deferrable介绍
deferrable的两个选项区别
deferrable表示该约束是可延迟验证的. 它有两个选项:
Initially immediate(默认): 立即验证, 执行完一个sql后就进行验证;
Initially deferred: 延迟验证, 当事务提交时或调用set constraint[s] immediate语句时才验证.
区别是: 事务提交时验证不通过, 则立即回滚事务; set constraint[s] immediate时只验证, 不回滚事务.
not deferrable与deferrable区别
区别就在于: “立即验证的可延迟约束” 是可以根据需要设置成 “延迟验证的可延迟约束”的, 而“不可延迟验证”是不能改变的.
- deferrable实例
建表
create table test1(a number(1) constraint check_a check(a > 0) deferrable
initially immediate,
b number(1) constraint check_b check(b > 0) deferrable
initially deferred);
正常插入,没问题
SQL> insert into test1 values(1, 1);
1 row inserted
检验立即验证:数据不能插入
SQL> insert into test1 values(-1, 1);
insert into test1 values(-1, 1)
ORA-02290: 违反检查约束条件 (MYHR.CHECK_A)
检验延迟验证:可以执行
SQL> insert into test1 values(1, -1);
1 row inserted
SQL> select * from test1;
A B
-- --
1 1
1 -1
提交延迟验证(commit):验证失败,自动回滚
SQL> commit;
commit
ORA-02091: 事务处理已回退
ORA-02290: 违反检查约束条件 (MYHR.CHECK_B)
提交延迟验证(set constraint immediate):验证失败,不回滚
SQL> insert into test1 values(1, -1);
1 row inserted
SQL> set constraint check_b immediate;
set constraint check_b immediate
ORA-02290: 违反检查约束条件 (MYHR.CHECK_B)
或者将所有的约束做修改:alter session set constraints = immediate;
或者:set constraints all immediate;
将延迟验证设置为立即验证:则在插入时出错
SQL> set constraint check_b immediate;
Constraints set
SQL> insert into test1 values(1,-1);
insert into test1 values(1,-1)
ORA-02290: 违反检查约束条件 (MYHR.CHECK_B)
- deferrable用途
物化视图
物化视图(快照),这是它的主要用途。这些视图会使用延迟约束来进行视图刷新。在刷新物化视图的过程中,可能会破坏完整性,而且将不能逐句检验约束。但到执行COMMIT时,数据完整性就没问题了,而且能满足约束。没有延迟约束,物化视图的约束可能会使刷新过程不能成功进行。
级联更新
当预测是否需要更新父/子关系中的主键时,它有助于级联更新。看一下实际的例子:
SQL> create table t(tno number(10) constraint pk_t_tno primary key, tname varchar2(20));
SQL> create table s(sno number(10) constraint pk_s_tno primary key, sname varchar2(20), tno number(10));
SQL> alter table s add constraint fk_s_tno foreign key (tno) references t(tno);
SQL> insert into t values(1,'yuechaotian');
SQL> insert into t values(2,'tianyuechao');
SQL> commit;
SQL> insert into s values(1,'stu_1', 1);
SQL> insert into s values(2,'stu_2', 1);
SQL> commit;
SQL> update t set tno=22 where tno=2;
SQL> update t set tno=11 where tno=1;
update t set tno=11 where tno=1
ORA-02292: 违反完整约束条件 (MYHR.FK_S_TNO) - 已找到子记录
SQL> select * from t;
TNO TNAME
----------- --------------------
1 yuechaotian
22 tianyuechao
SQL> select *from s;
SNO SNAME TNO
----------- -------------------- -----------
1 stu_1 1
2 stu_2 1
SQL> rollback;
SQL> alter table s drop constraint fk_s_tno;
SQL> alter table s add constraint fk_s_tno foreign key (tno) references t(tno) deferrable initially immediate;
```
------------------
```
SQL> select * from t;
TNO TNAME
----------- --------------------
1 yuechaotian
2 tianyuechao
SQL> select * from s;
SNO SNAME TNO
----------- -------------------- -----------
1 stu_1 1
2 stu_2 1
```
-----------
```
SQL> set constraint fk_s_tno deferred;
Constraints set
SQL> update t set tno=22 where tno=2;
SQL> update t set tno=11 where tno=1;
SQL> update s set tno = 11 where tno = 1;
SQL> commit;
```
----------------
```
SQL> select * from t;
TNO TNAME
----------- --------------------
11 yuechaotian
22 tianyuechao
SQL> select * from s;
SNO SNAME TNO
----------- -------------------- -----------
1 stu_1 11
2 stu_2 11
```
--------------------
```
SQL> select a.constraint_name, a.deferrable, a.deferred from user_constraints a where a.constraint_name like '%TNO%';
CONSTRAINT_NAME DEFERRABLE DEFERRED
------------------------------ -------------- ---------
FK_S_TNO DEFERRABLE IMMEDIATE
PK_S_TNO NOT DEFERRABLE IMMEDIATE
PK_T_TNO NOT DEFERRABLE IMMEDIATE
```
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