1Z0-051 35. Which two statements are true regarding constraints? (Choose two.)

35. Which twostatements are true regarding constraints? (Choose two.)

A. A table canhave only one primary key and one foreign key.

B. A table canhave only one primary key but multiple foreign keys.

C. Only theprimary key can be defined at the column and table levels.

D. The foreignkey and parent table primary key must have the same name.

E. Both primarykey and foreign key constraints can be defined at both column and table levels.

答案: BE  一个表只可以有一个主键，但是可以有多个外键。只有not null必须是列级约束，其他约束都可以做表级约束。

列级约束：carete   table    parent(c1 number  primary key); 

表级约束：create table child (c1  number primary key ,c2 number,constraint fk_c foreign key(c1)   references parent(c1));--列都定义完后用逗号隔开，进行表级约束定义。