MySql - Is primary key unique by default?

mysql primary-key unique-key
Roee Gavirel picture Roee Gavirel · Jun 18, 2015 · Viewed 22.4k times · Source

If I define a column as a primary key in MySql, is it also unique key by default or do I need to also define it as unique key (in case I want it to be unique)?

I saw this question What is the difference b/w Primary Key and Unique Key that explain the difference between the two but doesn't exactly answer my question. Does PK is UK by default or I need to explicitly define it.

Answer

Rahul Tripathi picture Rahul Tripathi · Jun 18, 2015

Primary key is always unique in every SQL. You dont have to explicitly define it as UNIQUE.

On a side note: You can only have onePrimary key in a table and it never allows null values. Also you can have only one primary key constraint in the table(as the point of creating a primary key is to uniquely identify the row in your table) but you can more than one unique key constraint in your table.

Example:

An employee details table having EmpID as Primary key and EmpPhoneNo as unique key.

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