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 run into the same question on one of the tables i did.

The problem was that the PK was supposed to be composed out of all the rows of the table all is well but this means that the table size will grow very fast with each row inserted.

I choose to not have a PK, but only have an index on the row i do the lookup on.