>I had thought that
>
>LEFT JOIN q3 ON pagcleanad = address AND pagbusines = trade_name
>
>was equivalent to
>
>RIGHT JOIN q3 ON address=pagcleanad AND trade_name=pagbusines
>
>but it clearly is not. So when we talk about left and right what exactly are we talking about? Apparently it is not the order of the equivalency. If we say LEFT JOIN Q3 we are supposed to preserve all the records on the left, but which table is on the left if it is not determined by the order of the equivalency?

It's based on the order that you list the tables, that is, which is to the left and which is to the right of the JOIN keyword. The ON is irrelevant to this.

Tamar