#unique-key

How to: Difference between Key, Primary Key, Unique Key and Index in MySQL

Difference between Key, Primary Key, Unique Key and Index in MySQL

When should I use KEY, PRIMARY KEY, UNIQUE KEY and INDEX?

Answer: Difference between Key, Primary Key, Unique Key and Index in MySQL

KEY and INDEX are synonyms.

You should add an index when performance measurements and EXPLAIN shows you that the query is inefficient because of a missing index. Adding an index can improve the…