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Candidate Key

Definition: A Candidate Key is a minimal set of columns that can uniquely identify each row in a table. A table can have one or more candidate keys, but only one is selected as the primary key.

Key Points:

  1. Must uniquely identify each row.
  2. Must be minimal — no extra columns.
  3. A table can have multiple candidate keys.
  4. One candidate key is chosen as the Primary Key.
  5. Remaining candidate keys become Alternate Keys.

Example: SQL Code with Comments

CREATE TABLE Employees (
    EmployeeID INT,                -- Candidate Key: can uniquely identify each employee
    Email VARCHAR(100) NOT NULL,   -- Candidate Key: also uniquely identifies an employee
    NationalID VARCHAR(20) NOT NULL -- Candidate Key: another unique identifier
);

-- Choosing Primary Key
ALTER TABLE Employees
ADD PRIMARY KEY (EmployeeID);      -- Primary Key selected from candidate keys

-- Alternate Keys (other candidate keys)
ALTER TABLE Employees
ADD UNIQUE (Email);               -- Email as alternate key
ALTER TABLE Employees
ADD UNIQUE (NationalID);          -- NationalID as alternate key

Key Examples from Code:

  • Candidate Keys: EmployeeID, Email, NationalID
  • Primary Key: EmployeeID
  • Alternate Keys: Email, NationalID

Relationship Summary:

  • Super Key → Candidate Key → Primary/Alternate Key → Unique Key
  • Candidate Key is a minimal super key.
  • Primary Key is one chosen candidate key.
  • Alternate Keys are other candidate keys.
  • All candidate and alternate keys are unique keys.