SQL · Theory

Primary Key vs Candidate Key

Theory

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Primary Key vs Candidate Key

A candidate key is any column set that could be the PK (unique + not null). You pick one candidate as the PRIMARY KEY. The others stay UNIQUE (alternate keys). roll and (aadhaar) might both be candidates — you pick roll as PK and UNIQUE on aadhaar.

This is theory (DBMS textbooks). In MySQL you implement: one PRIMARY KEY, other candidates as UNIQUE NOT NULL.

Trap — reciting ‘candidate key’ without an example table.

On the example next to this theory: Primary Key vs Candidate Key — keys and constraints protect data integrity; indexes speed lookups.

Diagram

  students.id  ← PK (unique)
       ▲
       │ FK
  marks.student_id

Exam tip

Candidates: roll or aadhaar. Pick one PK, UNIQUE the other.

Primary Key vs Candidate Key — sample query

CREATE TABLE authors (
  id INTEGER PRIMARY KEY,
  email TEXT UNIQUE NOT NULL
);
CREATE TABLE books (
  id INTEGER PRIMARY KEY,
  author_id INTEGER NOT NULL,
  title TEXT NOT NULL,
  FOREIGN KEY (author_id) REFERENCES authors(id)
);
INSERT INTO authors VALUES (1, 'a@x.com');
INSERT INTO books VALUES (1, 1, 'SQL Basics');
SELECT * FROM books;

Primary Key vs Candidate Key — keys and constraints protect data integrity; indexes speed lookups.

Short notes

DefCandidate = could be PK. One candidate is chosen as PK.
RuleOther candidates → UNIQUE NOT NULL.
TrapTheory without a students example.

CREATE TABLE authors ( id INTEGER PRIMARY KEY, email TEXT UNIQUE NOT NULL ); CREATE TABLE books ( id INTEGER PRIMARY KEY, author_id INTEGER NOT NULL, title TEXT NOT NULL, FOREIGN KEY (author_id) REFERENCES authors(id) ); INSERT INTO authors VALUES (1, 'a@x.com'); INSERT INTO books VALUES (1, 1, 'SQL Basics'); SELECT * FROM books;

Primary Key vs Candidate Key

Primary Key vs Candidate Key — sample query

Short notes

Questions

Primary Key vs Candidate Key

Primary Key vs Candidate Key — sample query

Short notes

Questions