SQL with Manoj

Let’s take an example of an Employee table:

CREATE TABLE Employee (
    EmployeeID, 
    EmployeeName, 
    DOB, -- Date of birth
    DOJ, -- Date of joining
    SSN, -- Social Security Number
    DeptID, -- Department ID
    MgrID -- Manager ID
)

1. Candidate Key: is the attribute/column or a set of attributes/columns in a relation/table that qualifies for uniqueness of each tuple/row. A relation/table can have one or more than one Candidate Keys. A Candidate key is also known as a minimal Super key.

Here in Employee table columns EmployeeID & SSN individually can maintain uniqueness in a table, thus are eligible for Candidate keys. The columns EmployeeName + DOB combined can also make up a Candidate Key, but there is a narrow chance that 2 Employees with same name can be born in same day.
 

2. Primary Key: is the Candidate key attribute/column that is most suited to maintain uniqueness in a table at the tuple/row level. More about PK.

Here in Employee table you can choose either EmployeeID or SSN column for a PK, EmployeeID is preferable choice because SSN is a secure (PII) value.
 

3. Alternate Key: are the other Candidate key attribute/columns that you didn’t choose as Primary key column.

Like if you choose EmployeeID as a PK then SSN would be the Alternate key.
 

4. Super Key: is a superset of Candidate key. If you add any other attribute/column to a Candidate Key then it become a Super Key.

Like EmployeeID + EmployeeName is a Super Key.
 

5. Composite Key: If a table do have a single column 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 in Employee table, then you can make EmployeeName + DOB as a Composite Primary Key. But still there can be a narrow chance of duplicate rows.
 

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