Emi Lu <emilu(at)encs(dot)concordia(dot)ca> writes:
> Just notice one small thing, and need your information about select
> select col1, col2
> from table1
> left join table2
> using (col1, col2)
> ;
> This query never complain about ambiguous columns of col1 and col2 in
> the select part.
That's because you used USING, which merges the similarly-named columns
into just one output column. RTFM, or any SQL book.
regards, tom lane