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what is candidate key in dbms

A candidate key in DBMS is a minimal set of attributes (columns) that uniquely identifies each row (tuple) in a table , and from which the primary key is chosen.

Quick Scoop: Simple Definition

  • A candidate key is any column or combination of columns that can uniquely identify every record in a table.
  • It must be unique (no two rows share the same candidate key value) and minimal (if you remove any attribute from it, it stops being unique).
  • A table can have multiple candidate keys, but only one of them is picked as the primary key.

Think of a class attendance sheet: Roll_No and Email might both uniquely identify a student. Both are candidate keys, but you choose only one as the primary key.

Key Properties (What makes a candidate key)

  1. Uniqueness
    • Every row must have a different value for the candidate key.
 * Example: In an `Employees` table, `EmployeeID` or `Email` can both be unique per employee.
  1. Minimality
    • You cannot remove any attribute from the candidate key and still uniquely identify rows.
 * If `{A, B}` is a candidate key, then neither `A` alone nor `B` alone should uniquely identify the row.
  1. Subset of Super Key
    • A candidate key is a minimal super key : it is a super key with no extra, redundant attributes.

Small Story-Style Example

Imagine a table Students:

Roll_No| ID| Name| Email
---|---|---|---
101| S01| Aayush| a@uni.edu
102| S02| Neha| n@uni.edu
103| S03| Ravi| r@uni.edu

  • Possible super keys could be: {Roll_No}, {ID}, {Email}, {Roll_No, ID}, {Roll_No, Email}, {ID, Email}, etc.
  • The candidate keys are the minimal super keys: {Roll_No}, {ID}, {Email} (each alone uniquely identifies a row).
  • From these, you pick one (say Roll_No) as the primary key , but ID and Email are still candidate keys.

How candidate key differs from other keys

Here is a quick comparison to avoid confusion:

html

<table>
  <tr>
    <th>Key Type</th>
    <th>What it is</th>
  </tr>
  <tr>
    <td>Super key</td>
    <td>Any set of attributes that can uniquely identify a row (may contain extra attributes).</td>
  </tr>
  <tr>
    <td>Candidate key</td>
    <td>Minimal super key with no redundant attributes; table can have many of these.</td>
  </tr>
  <tr>
    <td>Primary key</td>
    <td>One candidate key chosen by the designer as the main unique identifier.</td>
  </tr>
  <tr>
    <td>Alternate key</td>
    <td>Other candidate keys that were not chosen as primary key.</td>
  </tr>
</table>

All these definitions follow the standard relational model description where a candidate key is a minimal super key.

Why candidate keys matter (exam / interview POV)

  • They ensure data integrity by preventing duplicate rows.
  • They are the basis for normalization , helping to remove redundancy and anomalies.
  • They help classify attributes into prime (in some candidate key) and non-prime , which is used in higher normal forms like BCNF, 3NF, etc.

TL;DR:
A candidate key in DBMS is a minimal set of attributes that uniquely identifies each record , there can be multiple such keys per table, and the primary key is simply one selected candidate key.

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