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what is a candidate key

A candidate key is a minimal set of one or more columns in a table that can uniquely identify every row, and from which the primary key is chosen.

What Is a Candidate Key?

Think of a candidate key as a “best possible unique ID” for rows in a database table.

It is any attribute (or combination of attributes) that:

  • Uniquely identifies each row (no two rows share the same values in those columns).
  • Is minimal, meaning if you remove any column from it, it no longer uniquely identifies rows.

Because it satisfies these properties, every candidate key is a valid choice to become the primary key.

Key Properties (Simple View)

  • Uniqueness :
    Each value (or combination of values) in the candidate key is different for each row in the table.
  • Minimality :
    You can’t drop any column from the candidate key and still have uniqueness; no proper subset is itself a key.
  • One table, many candidates :
    A table can have multiple candidate keys, but only one of them is chosen as the primary key.

Example: In a Person table, both SSN and PassportNumber might each uniquely identify a person; both are candidate keys, but you choose just one as the primary key.

Candidate Key vs Other Keys (Quick Table)

[1][3][5] [3][5][7] [2][5][1] [9]
Concept What it means
Super key Any set of columns that uniquely identifies rows; may contain extra, unnecessary columns.
Candidate key A minimal super key: still unique, but with no redundant columns.
Primary key The candidate key actually chosen to be the main identifier for the table (must be unique and not null).
Alternate key Candidate keys that were not chosen as the primary key.

Why Candidate Keys Matter Today

Modern relational databases, from classic SQL setups to today’s large-scale systems, still rely on candidate keys to:

  • Enforce data integrity and avoid duplicate records.
  • Support normalization by clearly identifying dependencies between attributes.
  • Provide clean, reliable join paths between tables, especially in large analytics and transactional systems.

You’ll see current tutorials, blog posts, and Q&A threads (2023–2025 and beyond) repeatedly emphasize that identifying good candidate keys is a core skill in practical database design.

Mini Example

Imagine a Students table with columns: StudentID, Email, NationalID, Name, Semester.

  • If StudentID is unique for each student, it is a candidate key.
  • If Email is also guaranteed to be unique, then Email is another candidate key.
  • Name is not a candidate key because many students can share the same name.

The database designer picks one of the candidate keys (often StudentID) to be the primary key, and the others become alternate keys.

TL;DR : When someone asks “what is a candidate key,” they mean: a minimal, unique column set that can identify each row in a table and from which the primary key is chosen.

Information gathered from public forums or data available on the internet and portrayed here.