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5 votes
1. Primary key can’t contain Null value, but Candidate key value can have one NULL value.

2. If a column (part of candidate keys) contain one null value, then that column will remain as part of candidate key, but not be considered as primary key.

3. A table has only one column which can be represented as Primary Key, but on the other hand there can be more than one column which combines with each other and create Candidate Key (one of them would be considered as Primary key & rest would be Alternate Keys).
1 votes
1 votes
A candidate key is one which can be used as primary key that is not null and unique constraint both holding true. In short all primary keys are definitely candidate keys. That is one of the candidate keys is chosen as primary key.
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0 votes
There is no such restriction on the candidate key. It may or may not be NULL. Even the supersets of these keys can be NULL. But, the candidate key chosen as a primary key can never be NULL.
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0 votes
Candidate key can not be null.but it is possible that if candidate key has more than one attribute then the value of only one attribute (any attribute) can be null.

Candidate key uniquely identifies the tuples if null is allowed then many tuples will show by null value which Violet the definition  of candidate key

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