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

I've got a better example of a table that doesn't need a primary key - a joiner table. Say I have a table with something called "capabilities", and another table with something called "groups", and I want a joiner table that tells me all the capabilities that all the groups might have, so it's basicallly

create table capability_group
(  capability_id varchar(32),
    group_id     varchar(32));

There is no reason to have a primary key on that, because you never address a single row - you either want all the capabilities for a given group, or all the groups for a given capabilty. It would be better to have a unique constraint on (capabilty_id,group_id), and separate indexes on both fields.