Alexander Farber <alexander(dot)farber(at)gmail(dot)com> writes:
> why do these queries please return different results?
> FROM words_scores s
> LEFT JOIN words_games g ON s.gid = g.gid
> AND s.uid = 1
In this one, "s.uid = 1" is part of the LEFT JOIN condition, so it never
eliminates s rows. Rows not satisfying the condition will be
null-extended instead.
> FROM words_scores s
> LEFT JOIN words_games g USING(gid)
> WHERE s.uid = 1
In this one, it's part of the top-level WHERE, so it does eliminate s
rows.
regards, tom lane