1Z0-051 QUESTION 35 Primary key和foreign key的用法

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本文探讨了数据库表中的两种关键约束——主键(primary key)与外键(foreign key)。明确了每个表只能定义一个主键，但可以有多个外键。此外，文章还解释了这两种约束如何在列级和表级进行定义。

35. Which two statements are true regarding constraints? (Choose two.)
A. A table can have only one primary key and one foreign key.
B. A table can have only one primary key but multiple foreign keys.
C. Only the primary key can be defined at the column and table levels.
D. The foreign key and parent table primary key must have the same name.

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

答案：BE

解析：

A选项错误，同一个表里可以有一个primary key和多个foreign key；

B选项正确；

C选项错误，Primary key和foreign key都可在列级和表级定义；

D选项错误，foreign key和Primary key不一定要相同的名字；

E选项正确。

官方文档如下：

http://docs.oracle.com/cd/E11882_01/server.112/e40540/glossary.htm#CNCPT89290

primary key
The column or set of columns that uniquely identifies a row in a table. Only one primary key can be defined for each table.
primary key constraint
An integrity constraint that disallows duplicate values and nulls in a column or set of columns.

http://docs.oracle.com/cd/E11882_01/server.112/e40540/glossary.htm#CNCPT2047

foreign key

An integrity constraint that requires each value in a column or set of columns to match a value in the unique or primary key for a related table. Integrity constraints for foreign keys define actions dictating database behavior if referenced data is altered.