在表创建时,指定约束条件,也可在创建表后,再添加修改;约束条件分为列级或者表级;约束延期是一个对主键、唯一键等约束进行延
1)约束constraints:在表上强加的约束条件,以保证数据的完整性;防止关联表的数据丢失;
常见的约束类型:
- NOT NULL
- UNIQUE
- PRIMARY KEY
- FOREIGE KEY
- CHECK
在表创建时,指定约束条件,,也可在创建表后,再添加修改;约束条件分为列级或者表级;
REATE TABLE [schema.]table
(column datatype [DEFAULT expr]
[column_constraint],
...
[table_constraint][,...]);
2)添加约束
ALTER TABLE table
ADD [CONSTRAINT constraint] type (column);
3)删除约束
ALTER TABLE table_name DROP CONSTRAINT constraint_name
4)重名约束
alter table table_name rename constraint old_name to new_name
5)约束有效/无效
alter table table_name enable/disable constraint constraint_name;
约束延期是一个对主键、唯一键等约束进行延迟检查的功能。可以使用该功能将约束检查延迟到事务提交时,而不是在进行DDL操作之后立即进行检查。所以对约束检查也有这两个选项:延迟和立即;
SQL> create table aaa
2 (
3 c1 number constraint aaa_pk primary key deferrable initially immediate,
4 c2 number constraint c2_ck check(c2>10) deferrable initially immediate
5 );
Table created.
SQL> set constraints c2_ck deferred;
Constraint set.
SQL> insert into aaa value (1,1);
insert into aaa value (1,1)
*
ERROR at line 1:
ORA-00928: missing SELECT keyword
SQL> insert into aaa values (1,1);
1 row created.
SQL> commit;
commit
*
ERROR at line 1:
ORA-02091: transaction rolled back
ORA-02290: check constraint (SCOTT.C2_CK) violated
提交时就会约束检查,即延迟检查;
6)查看约束
user_constraints
user_cons_columns

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