Re: wrong field in example

Just noticed you replied to me only.  Send replies to the list.  You can leave individuals cc'd.  The convention here is also to inline your replies - top-posting is undesirable.

In any case, in order to write "select *" you have to group on both x and y at the same time.  If you group on either column only, the query will fail.  So the choice of x versus y is immaterial.  Having grouped on x the text is explaining that the failure to also group on y is a problem.