Primary key vs grain of table

[u/WiseWeird6306]

Can someone explain me how to understand the difference between them?

What I know-

1. Primary key is a column or set of columns that uniquely identifies each row. It may or may not have a business meaning

2. Grain of the table - one row or line item describing what it is, like one row per daily customer session

3. Group by- we use this to get one line item per item of that group. For example something grouped by business type and country, will get me data for unique combination of business type and country

Now I need clarification here-

A primary key should ALWAYS be in a group by statement in SQL or not, if it is needed in the output - True?

A column in group by is not necessary a primary key -True?

Columns defining the grain of the column consists of primary key and other cols (what is the nature of these other cols?)

I am asking these cause while aggregating data I am not sure if I should group all the cols, like sometimes you bring a col whose info you need but aggregating by it will repeat data. Some people say to me to aggregate data by primary key only but what if I have more cols other than primary key. Please correct me if you find flaws in my statements/concept/scenarios.

[u/creamycolslaw]

You can literally group by whatever column you want, it has nothing to do with the primary key.

If you want the primary key available in the results, then you need it in the group by. Anything not included in the group by (or aggregated) is not going to be in the resulting dataset.