FresherFresher (0–1 yrs)CodingSQLInfosysAccentureAmazon

PRIMARY KEY vs UNIQUE

Difference between PRIMARY KEY and UNIQUE constraints?

Answers use simple, clear English.

Quick interview answer

PRIMARY KEY uniquely identifies a row, disallows NULL, and usually implies a clustered index. UNIQUE also enforces uniqueness but may allow one/multiple NULLs depending on the engine, and a table can have many UNIQUE constraints but only one PRIMARY KEY.

Detailed answer

Code example

CREATE TABLE users (
  id INT PRIMARY KEY,
  email VARCHAR(255) UNIQUE NOT NULL
);

Practice code · sql (view only · no execution)

CREATE TABLE users (
  id INT PRIMARY KEY,
  email VARCHAR(255) UNIQUE NOT NULL
);

#sql#basics