Good Question, Any sql will be having only one primary key for one table, if you want to make another keys, you can add unique keys to the columns and you can make it as a primary(with 2 or more columns).
If we use auto_increment in MySQL, it assumes the column as a Primary Key and asks us to add Primary Key to that column.
The 'key' is there to 'unlock' all other fields in the table. It 'auto increments' with a 'unique id' so that records are kept separate and tidy and so that you can access and refer to and update specific records.
a key or primary key is required for anything that is 'auto incremented'
so you can 'unlock' the rest of the table. The field should be called
something like "table_id" and use "auto increment" and be "primary key"
or you will get errors for sure.