About index and primary key in SQL?
Yes, you can assume it is a standard rule that RDBMSes require an index for the primary key.
Mainly, the database engine needs the index to enforces data uniqueness for the primary key, and to do this quickly requires an index.
Have a look at this
If a PRIMARY KEY constraint is created for a database table and no clustered index currently exists for that table, SQL Server automatically creates a clustered index on the primary key
In MSSQL server, creating a primary key will normally create a clustered index by default .