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。