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

Technically, you can declare such a table.

But in your case, the time should be made the PRIMARY KEY, since it's probably wrong to have different temperatures for the same time and probably useless to have same more than once.

Logically, each table should have a PRIMARY KEY so that you could distinguish two records.

If you don't have a candidate key in you data, just create a surrogate one (AUTO_INCREMENT, SERIAL or whatever your database offers).

The only excuse for not having a PRIMARY KEY is a log or similar table which is a subject to heavy DML and having an index on it will impact performance beyond the level of tolerance.