ORACLE约束(constraint)：对象的强制规定
5种约束：
NOT NULL 非空 NN
UNIQUE 唯一 UK
PRIMARY KEY 主键 PK
FOREIGN KEY 外键 FK
CHECK 条件 CK
约束创建：
1)创建对象时定义
2) 创建对象后定义
3) 约束有名字：默认：SYS_C0, 系统推荐：表名_列名_约束类型
create table scott.tab
drop table scott.me_stu purge;
create table scott.me_stu(
id varchar2(20) constraints stu_id_pk primary key,
nam varchar2(32) constraints stu_nam_uk unique,
age number(10) constraints stu_sal_ck check(age>0),
notes varchar2(32) constraints stu_not_nn not null
) compress nologging;
或
drop table scott.me_stu purge;
create table scott.me_stu(
id varchar2(20),
nam varchar2(32),
age number(10) ,
notes varchar2(64),
constraints stu_id_pk primary key(id),
constraints stu_nam_uk unique(nam),
constraints stu_sal_ck check(age>0),
constraints stu_notes_nn check(notes is not null)
) compress nologging;
alter table scott.me_stu move compress;
--外键
--参照完整性约束
--限制(Restrict)。不允许进行修改或删除操作。若修改或删除主表的主键时，如果子表中存在子记录，系统将产生一个错误提示。这是缺省的参照完整性设置。
--置空(Set Null)。如果外键列允许为空，若修改或删除主表的主键时，把子表中参照的外键列设置为空值(NULL)。
--置为缺省(Set Default)。如果指定了缺省值，若修改或删除主表的主键时，把子表中参照的外键设置为缺省值(Default)。
--级联(Cascade)。把主表中主键修改为一个新的值时，相应修改子表中外键的值；或者删除主表中主键的记录时，要相应删除子表中外键的记录。
drop table scott.me_score purge;
create table scott.me_score (
sid varchar2(20) constraint score_sid_fk references scott.me_stu(id),--constraint score_sid_fk references scott.me_stu(id) on delete cascade,
cid varchar2(20),
score number(10,2)
) nologging;
或
drop table scott.me_score purge;
create table scott.me_score (
sid varchar2(20),
cid varchar2(20),
score number(10,2),
--constraint score_sid_fk foreign key(sid) references scott.me_stu(id)
constraint score_sid_fk foreign key(sid) references scott.me_stu(id) on delete cascade
) nologging;
--查询约束
select * from dba_constraints where table_name like '%me_stu%';
select * from dba_cons_columns where table_name like '%me_stu%';
--删除约束
alter table scott.me_stu drop constraints stu_nam_uk;
alter table scott.me_score drop constraints score_sid_fk;
--添加约束
alter table scott.me_stu modify(nam varchar2(32) constraint stu_nam_nn unique);
alter table scott.me_stu modify(notes varchar2(32) constraint stu_not_nn not null);
alter table scott.me_stu add constraint stu_nam_uk unique(nam);
alter table scott.me_score add constraint score_sid_fk foreign key(sid) references scott.me_stu(id) on delete cascade;
--外键对DML影响
INSERT：
1)子表对父表无影响
2)插入子表外键时,父表必须存在
DELETE：
1)对子表DELETE操作对父表无影响
2)对父表DELETE操作须先对子表删除
UPDATE：
1)对父表和子表都有影响
2)对父表如果子表引用则不能更新
3)对子表UPDATE操作时,外键在父表必须存在
--外键对DDL影响
1)对子表没影响，对父表如果子表引用则不能删除
--为了消除影响可以在建立约束时添加子句：
on delete set null 主表删除时,子表置空
on delete cascade 主表删除时,子表级联删除
--禁用与启用约束
alter table scott.me_score disable constraint score_sid_fk;
alter table scott.me_score enable constraint score_sid_fk;
约束(constraint)状态：
ENABLED/DISABLED
VALIDATED/NOVALIDATED
DEFERRABLE/NON-DEFERRABLE
DEFERRED/IMMEDIATE
RELY/NORELY
1)DEFERRABLE/NON-DEFERRABLE,DEFERRED/IMMEDIATE
deferrable: constraint如果被定义成deferrable那么constraints可以在deferred和imediate两种状态相互转换
not deferrable: constraint默认是not deferrable,同initially immediate,不能在deferred和imediate两种状态相互转换
deferred: 意味着constraint将被延迟即在transaction过程中使constraint失效,等到如果transaction commit时transaction会变成immediate
immediate: 意味着constraint在transaction过程中使constraint一直有效
deferrable initially immediate: 允许将constraint再改为initially deferred
deferrable initially deferred: 允许将constraint再改为initially immediate
drop table scott.test purge;
create table scott.test
( x number constraint check_x check (x > 0) deferrable initially immediate,
y number constraint check_y check (y > 0) deferrable initially deferred
)nologging;
或
alter table scott.test add constraint check_x (x > 0) deferrable initially immediate;
alter table scott.test add constraint check_y (y > 0) deferrable initially deferred;
insert into scott.test values ( 1,1 );
commit;
--initially immediate:在transaction过程中使constraint一直有效
insert into scott.test values (-1,1 );
ERROR at line 1:
ORA-02290: check constraint (SCOTT.CHECK_X) violated
--转换initially immediate=>deferred
set constraint scott.check_x deferred;
insert into scott.test values (-1,1 );
1 row created.
commit;
ERROR at line 1:
ORA-02091: transaction rolled back
ORA-02290: check constraint (SCOTT.CHECK_X) violated
--initially deferred:constraint被延迟,transaction commit时transaction会变成immediate
insert into scott.test values ( 1,-1 );
1 row created.
commit;
ERROR at line 1:
ORA-02091: transaction rolled back
ORA-02290: check constraint (SCOTT.CHECK_Y) violated
--转换initially deferred=>immediate
set constraint scott.check_y immediate;
insert into scott.test values ( 1,-1 );
ERROR at line 1:
ORA-02290: check constraint (SCOTT.CHECK_Y) violated
1)enable/disable validate/novalidate
enable/disable: 对未来的数据有约束/无约束
validate/novalidate: 对已有的数据有约束/无约束
启用约束:
enable( validate) :启用约束,创建索引,对已有及新加入的数据执行约束.
enable novalidate :启用约束,创建索引,仅对新加入的数据强制执行约束,而不管表中的现有数据.
禁用约束:
disable( novalidate):关闭约束,删除索引,可以对约束列的数据进行修改等操作.
disable validate :关闭约束,删除索引,不能对表进行 插入/更新/删除等操作.
注意：如果加约束到一个大表，那么ORACLE会LOCK这个表，然后SCAN所有数据，来判断是否符合CONSTRAINT的要求，在繁忙的系统里显然是不合适的。
所以用enable、novalidate比较合适，因为ORACLE仅仅会LOCK表一小段时间来建立CONSTRAINT，当CONSTRAINT建立后再VALIDATE，这时检验数据是不会LOCK表
alter table scott.me_score disable validate constraint score_sid_fk;
alter table scott.me_score enable novalidate constraint score_sid_fk;
