047 exam c 20-end

22.which three statements are ture

a.only one long column can be used per table

oracle只允许每个表中有一个long类型的列

b.a timestamp data type column stores only time values wqith fractinal secondss

timestamp类型可保存的是时间戳，包括年，月，日，时，分，秒，秒最大精确到小数带点后9位，默认值是6位

35.user OE,the owner of the orders table ,issues the following command

grant select .insert on orders to hr with grant option

the user hr issues the follwoing command

geant select on oe.orders to scott

then.oe issues the follwoing command 

revoke all on orders from hr

which statement is correct

a.the user scott loses the privilege to select rows from or.orders

授予hr对表orders的SELECT,INSERT权限,并且授予hr将这些权限授予别人的权限

The user HR issues the following command:

GRANT SELECT ON oe.orders TO scott
授予scott对表oe.orders的SELECT权限

Then, OE issues the following command:

REVOKE ALL ON orders FROM hr
撤销hr对表orders的所有权限

WITH GRANT OPTION只能在赋予 object privilege 的时使用,撤销时有连带效果oe>>hr>>scott

39.which two statements are true regarding constraints?

a.a foreign key cannot contain null values

外键是另一张表的主键或者唯一列，唯一列就允许为空

b.a column with the unique contarints can contain null 

yes

C. A constraint is enforced only for the INSERT operationon a table.
可以将约束设置成可延迟的，在commit时执行检查，约束会在除select的所有dml语句执行检查，不单单是insert

D. A constraint can be disabled even if the constraintcolumn contains data.(right)
约束可以声明失效和约束所在列是不是有数据没有关系，所以即使有数据也能使约束失效

E. All the constraints can be defined at the column levelas well as the table level.
约束里的非空约束和默认值约束是列级约束，其他既是列级约束又是表级约束