referential integrity

Question

select true statements:

foreign key may be null.

primary key may be null.

none

Answer 1

Foreign key may be NULL but can not have mismatched value from its corresponding primary/unique key Ref Integrity

Primary key can not have NULL or duplicate value    Entity Integrity

hence first statement is true

Comments

How foreign key can be null? It refers the values that forms the primary key for one table. Now, can NULL be one of the values for the primary key in this referenced table?

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If you do ON DELETE SET NULL for foreign key then on deletion of a primary key value then corresponding foreign key value will be set to NULL.Hence only 1st statement is true

But primary key is never NULL and unique.

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it is quite possible and natural for a foreign key to be Null e.g bank a/c no. is P.K in some table  like Bank

and bank a/c no. is F.K. in some other table like student  derving its value from Bank table   now a student may or may not have a bank a/c if it have then it must be defined in bank table it not then it is NULL

Answer 2

Primary key can't be Null in any case otherwise there would not be any use of it.But foreign key can be Null. And it refers to the primary key in the first table.