Candidate Key is a column or combination of columns, which can be a
Primary key for the Table.
There may be multiple
Candidate Keys In a Table.
Any
Candidate Key can be a
Primary Key.
Unique Key is a column which has unique value for each row, but it also allow nulls (
Primary Key doesn't).
Any confusions, feel free to ask.
Read -
SQL SERVER – Difference Between Candidate Keys and Primary Key[
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