SQL · Theory

Primary Key vs Foreign Key

Theory

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Primary Key vs Foreign Key

PRIMARY KEY is this table’s identity (students.roll). FOREIGN KEY is a pointer in another table (marks.roll) that must match a parent PK/UNIQUE. Parent first, then child.

You can query without declaring an FK, but then the database will not stop orphans. Declare the FK.

Trap — putting the FK on the parent instead of the child.

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

Diagram

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

Exam tip

students.roll PK. marks.roll FK → students.roll.

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

Short notes

DefPK identifies a row. FK points at a parent PK.
RuleFK lives on the child table.
TrapFK on the parent table.

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 Foreign Key

Primary Key vs Foreign Key — sample query

Short notes

Questions

Primary Key vs Foreign Key

Primary Key vs Foreign Key — sample query

Short notes

Questions