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原创 Linux操作系统 作者:jbymy2000 时间:2012-03-14 14:27:26 0 删除 编辑
47. Which two statements are true about the primary key constraint in a table? (Choose two.)
A) It is not possible to disable the primary key constraint.
B) It is possible to have more than one primary key constraint in a single table.
C) The primary key constraint can be referred by only one foreign key constraint.
D) The primary key constraint can be imposed by combining more than one column.
E) The non-deferrable primary key constraint creates an unique index on the primary key column if it is not already indexed.
答案:DE
当定义主键约束时,默认情况下oracle会自动基于唯一约束建立唯一索引,并且索引名与约束名完全一致,一张表只能有一个主键约束。一个主键约束可以由多个列组成。

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