Difference between primary key and unique key in MySQL explains differences between Primary key and Unique constraint in MySQL or MairaDB
Below are the differences between Primary Key and Unique Key
Primary Key does not accept Null values but Unique Key column can accept Null values.
There is only One Primary Key per table but You can have multiple Unique Keys per Table
By default it added cluster index when you create Primary key, for Unique Key it creates Non-cluster index.
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