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I'll take example of an Employee
1. Candidate Key:
are individual columns in a table that qualifies for uniqueness of all the rows. Here in Employee table EmployeeID & SSN are Candidate keys.
2. Primary Key:
is the columns you choose to maintain uniqueness in a table. Here in Employee table you can choose either EmployeeID or SSN columns, EmployeeID is preferable choice, as SSN is a secure value.
3. Alternate Key:
Candidate column other the Primary column, like if EmployeeID is PK then SSN would be the Alternate key.
4. Super Key:
If you add any other column/attribute to a Primary Key then it become a super key, like EmployeeID + FullName is a Super Key.
5. Composite Key:
If a table do have a single columns that qualifies for a Candidate key, then you have to select 2 or more columns to make a row unique. Like if there is no EmployeeID or SSN columns, then you can make FullName + DateOfBirth as Composite primary Key. But still there can be a narrow chance of duplicate row.