（31 ） 查询尚未归还书的图书编号和借书日期，正确的 SQL 语句是

A ） SELECT 图书编号 , 借书日期 FROM 借阅 WHERE 还书日期 =" "

B ） SELECT 图书编号 , 借书日期 FROM 借阅 WHERE 还书日期 = NULL

C ） SELECT 图书编号 , 借书日期 FROM 借阅 WHERE 还书日期 IS NULL

D ） SELECT 图书编号 , 借书日期 FROM 借阅 WHERE 还书日期

Which statement is true regarding virtual private catalogs?（）

A、A virtual private catalog owner can create a local stored script, and have read/write access to a global stored script.

B、The virtual private catalog owner cannot create and modify the stored scripts.

C、The set of views and synonyms that make up the virtual private catalog is stored in the schema of the RMAN recovery catalog owner.

D、To perform most of the RMAN operations, the virtual catalog owner must have the SYSDBA or SYSOPER privilege on the target database

（7）已有查询文件queryone.qpr，要执行该查询文件可使用命令【7】。

以下关于“试图”的正确描述是

A)视图独立于表文件

B)视图不可更新

C)视图只能从一个表派生出来

D)视图可以删除

Which of these appropriately describes the results of a manual SQL Tuning Advisor task?（）

A、A list of SQL statements and recommendations for tuning

B、A list of SQL statements that have been tuned by the Advisor, with before and after metrics

C、Graphs showing the actual performance improvement made by the Advisor after it implemented the recommended changes

D、All of the above

虚电路在传输分组时建立__________连接。