1Z0-051 QUESTION 28 关于约束

QUESTION 28

Which two statements are true regarding constraints? (Choose two.)

A. A foreign key cannot contain NULL values.
B. A column with the UNIQUE constraint can contain NULL values.
C. A constraint is enforced only for the INSERT operation on a table.
D. A constraint can be disabled even if the constraint column contains data.

E. All constraints can be defined at the column level as well as the table level.

答案：BD

A选项错，外键可以是空。

SQL> create table test1(
  2  t1 varchar(10) null);

Table created.

SQL> insert into test1 values('')
  2  ;

1 row created.

SQL> select * from test1;

T1
--------------------


SQL> create table test2(id2 varchar2(10));

Table created.

SQL> alter table test2 add constraint pk primary key (id2)
  2  ;

Table altered.

SQL> alter table test1 add constraint fk_test foreign key (t1) references test2(
id2);

Table altered.

B选项对，唯一约束可以是null值。

SQL> create table test (t1 varchar2(10) null)
  2  ;

Table created.

SQL> alter table test add constraint qk_test unique(t1);

Table altered.

C选项错，约束可以作用于INSERT，UPDATE，DELETE语句。

D选项对，disabled只是暂时的，和数据无关。

E选项错，约束能是在列级定义，不能在表级定义。