Nullable integer as part of composite primary key

Question

I want to store phone numbers as integers and I've created this SQL:

DROP TABLE `user_id_phone`;
CREATE TABLE `user_id_phone`
(
    `user_id`      int     unsigned  NOT NULL,
    `country_code` smallint unsigned NOT NULL,
    `number`       bigint unsigned   NOT NULL,
    `ext`          smallint unsigned NULL,
    PRIMARY KEY (`country_code`, `number`, `ext`)    
) ENGINE = InnoDB

But my server (10.4.14-MariaDB) creates field ext as 'NOT NULL':

`ext` smallint(5) unsigned NOT NULL,

if ext is a part of the composite primary key:

PRIMARY KEY (`country_code`, `number`, `ext`) 

In case of ext is not a part of the primary key:

PRIMARY KEY (`country_code`, `number`),

the field DDL is right after creation:

  `ext` smallint(5) unsigned DEFAULT NULL,

Why?

Answer 1

Why?

Because you can't have NULL values in a primary key. Why? Because the SQL standard says so:

If the <unique specification> specifies PRIMARY KEY, then for each <column name> in the explicit or implicit <unique column list> for which NOT NULL is not specified, NOT NULL is implicit in the <column definition>.

This is also described in documentation.

PS. Phone numbers are not integers and should not be stored as such. They are character sequences that often, but not always, contain mostly digits.