MySQL + SQL · Lesson 1

Dbms Keys Explained

All Keys in One Place

This page summarizes every key type in DBMS with a single clear example.

Key	One-line Meaning
Super key	Any unique-identifying column set
Candidate key	Minimal super key
Primary key	Chosen main identifier (unique, not null)
Alternate key	Candidate keys not chosen primary
Foreign key	Refers to another table's primary key
Composite key	Made of multiple columns

students(roll_no PK, admission_no UNIQUE, name)
marks(roll_no FK, subject, score, PRIMARY KEY(roll_no, subject))

marks uses a COMPOSITE primary key (roll_no + subject) and roll_no is a FOREIGN key.

Key	एक-पंक्ति मतलब
Super key	कोई भी unique-पहचान column set
Candidate key	Minimal super key
Primary key	चुना गया मुख्य identifier (unique, not null)
Alternate key	जो candidate keys primary नहीं चुनीं
Foreign key	दूसरी table की primary key को refer करती है
Composite key	कई columns से बनी

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