Primary key vs Unique key vs Index

 

Type	Purpose
Primary key	Uniquely identifies each row
Unique key	Prevents duplicate values
Index	Improves lookup/query performance

Example:

$table->id();                         // Primary key
$table->string('email')->unique();    // Unique + index
$table->index(['status', 'created_at']); // Normal composite index

A primary key is unique and non-null. A unique constraint prevents duplicate values but is not necessarily the table's identity.

Real-project scenario

For users:

id          -> PRIMARY KEY
email       -> UNIQUE
status      -> INDEX

If the application frequently executes:

SELECT * FROM orders
WHERE user_id = 10
ORDER BY created_at DESC;

an index involving user_id and potentially created_at can improve performance.