Question

can cadidate key may be null?

Comments

Null values are unique because they are unknown values. Don't think like if they are unique it then doesn't matter. You must have at least one CK , whose all entries are non nullentries.I mean there must be a primary key in a relation (specification is optional albeit)

a relation like R doesn't exist in Relational database

R (A          B)

     2          NULL

    NULL      7

    4            2

in last comment the example i provided , is just to tell you how can CK be NULL. in real scenario there must be column who can play part as primary key ,if we want to specify.

---

Thanks sir,

Got it

---

reference korth chapter 4 

   4.4.3 Unique Constraint
SQL also supports an integrity constraint:
unique (Aj1 , Aj2, . . . , Ajm )
The unique specification says that attributes Aj1 , Aj2, . . . , Ajm form a candidate
key; that is, no two tuples in the relation can be equal on all the listed attributes.
However, candidate key attributes are permitted to be null unless they have
explicitly been declared to be not null. Recall that a null value does not equal
any other value. (The treatment of nulls here is the same as that of the unique
construct defined in Section 3.8.4.)

Answer 1

Yes candidate key can be null but primary key can't contain null .

Answer 2

This question is causing confusion. Because we have have learnt that – 

1. Candidate Key = The minimal set of attributes which can uniquely identify a tuple is known as  candidate key.
2. Primary Key  = There can be more than one candidate key in relation out of which one can be chosen as the primary key.

• Primary key can not contain “null” values.

But note that, candidate key can contain null values. It is just that in that case, that candidate key can not be chosen as primary key. We need to chose some other candidate key as primary key, which can not contain “null” values.