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).