老父举牌寻目击者,过温保护,西安上门按摩
q6.which two statements are true regarding constraints (choose two)
a. a constraint is enforced only for an insert operation on a table.
b. a foreign key cannot contain null values.
c. the column with a unique constraint can store nulls.
d. you can have more than one column in a table as part of a primary key.
correct answer: cd
对于 a 选项,foreign key 可以包含 null
对于 c 选项,constaint 在 delete,update 的时候也会应用。
对于 e 选项,not null 不能在表级别定义,只能在列级别定义。
如对本文有疑问,请在下面进行留言讨论,广大热心网友会与你互动!! 点击进行留言回复
Oracle字符串函数length、upper、lower、initcap、 concat、instr、replace
网友评论