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Database Keys Use Cases

📅 April 08, 2026

Primary Key

  • Always Unique
  • Never NULL
  • Only one per table
  • Required per table

Candidate Key

  • Uniquely identify a row
  • Non-key attributes
  • The primary key is chosen from these

Super Key

  • A set of attributes that uniquely identify a row but is not minimal
    • StudentID → superkey
    • (StudentID, Name) → still a superkey
    • (StudentID, Email, Age, Height) → still a superkey = with Student Id considered as unique

Natural Key

  • A key that already exists in the real world

Surrogate Key

  • A system-generated key with no real-world meaning
  • A made-up ID just for the database

Intelligent Key

  • A key that encodes meaning inside it
    • EMP2024‑001 (year + type + sequence)
    • PH‑MNL‑0001 (country + city + sequence)

Alternative Key

  • A candidate key that was NOT chosen as the primary key
  • Unique and Valid

🧩 Summary Table

Key Type Meaning Shortcut Memory Example
Primary Key The officially chosen unique identifier of the table. The chosen one UserID
Candidate Key Any attribute (or combo) that can uniquely identify a row. All possible unique identifiers Email, Username
Super Key Any attribute set that uniquely identifies a row (even with extras). Unique with extra baggage (UserID, Email)
Natural Key A key that comes from real-world data. Already exists in real world PlateNumber, SSN
Surrogate Key A system-generated key with no real-world meaning. Made-up ID for the database Auto-increment ID, UUID
Intelligent Key A key that encodes meaning inside it. ID with built-in meaning EMP-MNL-2024-001
Composite/Compound Key A key made of two or more columns acting together as one identifier. Multiple columns = one key (StudentID, CourseID)
Alternative Key A candidate key not chosen as the primary key. The runner-up Email if UserID is PK