Primary Key vs Foreign Key — SQL Technical Interview Questions & Answers

Primary Key vs Foreign Key

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.

Technical Interview Tip: students.roll PK. marks.roll FK → students.roll.

Code Examples for Primary Key vs Foreign Key

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.

How to Explain Primary Key vs Foreign Key to an Interviewer

Where do you put the FOREIGN KEY?

On the child table Points at parent PK