Foren / Craftwork / Server-side / Back to PHP MySQL: A question!

I've gone back to trying to re-learn and learn php/sql .... I have a question about how the mySQL db is being called in a situation like say:

sql query;
assign array var {
assigning results to var from sql array
      sql query;
      assign array var {
      assigning results to var from sql array
      }
}

My question is, is the second sql query being looped.. like for each result... is it a new query for every row from the query? Or is it being queried once after the first query and only grabbing relevent rows?

I know this question is really basic/general but I assumed it was like a foreach loop (on the second query) and if so, it can't be good right?

Thanks!