SQL: Where should you define the primary key


I'm creating a database with several sql files 1 file creates the tables. 1 file adds constraints. 1 file drops constraints.

The primary is a constraint however I've been told by someone to define your primary key in the table definition but not given a reason why.

Is it better to define the primary key as a constraint that can be added and dropped or is it better to do it in the table definition.

My current thinking is to do it in the table definition because doing it as a removable constraint could potentially lead to some horrible issues with duplicate keys. But dropping constraints could lead to serious issues anyway so it is expected that if someone did drop the primary key, they would have taken appropriate steps to avoid problems as they should have for any other data entry

A primary key is a constraint, but a constraint is not necessarily a primary key. Short of doing some major database surgery, there should never be a need to drop a primary key, ever.

Defining the primary key along with the table is good practice - if you separate the table and the key definition, that opens the window to the key definition getting lost or forgotten. Given that any decent database design utterly depends on consistent keys, you don't ever want to have even the slightest chance that your primary keys aren't functioning properly.