Can a database table be without a primary key?

Question

Can anyone tell me if a table in a relational database (such as MySQL / SQL SERVER) can be without a primary key?

For example, I could have table day_temperature

Answer 1

The time would then become your primary key. It will help index that column so that you can query data based on say a date range. The PK is what ultimately makes your row unique, so in your example, the datetime is the PK.