1Z0-051 QUESTION 29 关于约束

QUESTION 29

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

A. A foreign key cannot contain NULL values.

B. The column with a UNIQUE constraint can store NULLS .

C. A constraint is enforced only for an INSERT operation on a table.
D. You can have more than one column in a table as part of a primary key.

答案：BD

ABC的解析查看QUESTION 28 ；

D选项正确，一个表可以有多个主键：

SQL> create table test (
2 id number(1),
3 t1 varchar2(20));

Table created.

SQL> alter table test add constraint pk_test primary key(id,t1);

Table altered.