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。