title: sql joins as venn diagram
body: <p>I've had trouble understanding joins in sql and came upon this image which I think might help me. The problem is that I don't fully understand it.  For example, the join in the top right corner of the image, which colors the full B circle red and but only the overlap from A. The image makes it seem like circle B is the primary focus of the sql statement, but the sql statement itself, by starting with A (select from A, join B), conveys the opposite impression to me, namely that A would be the focus of the sql statement. </p>
  
  <p>Similarly, the image below that only includes data from the B circle, so why is A included at all in the join statement?</p>
  
  <p>Question: Working clockwise from the top right and finishing in the center, can someone provide more information about the representation of each sql image, explaining </p>
  
  <p>a) why a join would be necessary in each case (for example, especially in situations where no data's taken from A or B i.e. where only A or B but not both is colored)</p>
  
  <p>b) and any other detail that would clarify why the image is a good representation of the sql</p>
  
  <p><img src="https://i.stack.imgur.com/UI25E.jpg" alt="sql join diagram"></p>