Add Primary Key
There're 3 basic ways to create a primary key for a table.
1. System-Generated Name
You don't have to provide a name for the primary key, if you have no idea about the name of the primary key, you can let the database system generate one for you.
SQL> alter table hr.job_history add primary key (employee_id, start_date);
The system-generated name may look like this:
To know the actual constraint name, you may refer to How to Check Primary Key of a Table.
2. User-Defined Name
You may also provide a name for the primary key constraint according to your naming convention.
SQL> alter table hr.job_history add constraint EMP_ID_ST_DT_PK primary key (employee_id, start_date);
Sometimes, I found user-defined names could distract developers from the correct way of troubleshooting.
3. Using an Existing Index
First of all, I created an unique index for later use.
SQL> create unique index hr.EMP_ID_ST_DT_PK on hr.job_history (employee_id, start_date);
Then I use the index as a base for the primary key by USING INDEX clause while adding the constraint.
SQL> alter table hr.job_history add constraint EMP_ID_ST_DT_PK primary key (employee_id, start_date) using index hr.EMP_ID_ST_DT_PK;
Please note that, the name of the primary key constraint can be different from the unique index.
For your reference, there're more examples about how index associate with a constraint in the documentation.
To revert the action of adding primary key to a table, you may like to know how to drop the primary key from a table.
One question for you: Can we create a non-unique primary key?