D. Richard Hipp schrieb: > On Jan 9, 2009, at 3:16 PM, Lukas Haase wrote: >> SELECT t.topic, t.length >> FROM printgroup AS pg1 >> LEFT JOIN printgroup AS pg2 ON pg1.printgroup = pg2.printgroup >> LEFT JOIN topics AS t ON t.topicID = pg2.topicID >> LEFT JOIN topic_ids AS ti ON ti.topicID = pg1.topicID >> WHERE ti.topic_textID = 'XXXX' >> ORDER BY pg2.topicID ASC; > > You seem very fond of using LEFT JOINs in places where they do not > make good sense.
Yes, I started with mySQL 3 many years ago. At the beginning I only knew about LEFT JOINs and used them. Now I think I also know the other types of JOINs but I still use LEFT JOINs very often, just by habit. And with mySQL I never had performance problems with them. > What is it that you think a LEFT JOIN does? (A LEFT JOIN B) joins together table A and B while all records are taken from A and only records that match both are takes from B. If a record from A has no corresponding data in B, the values are NULL. > How is > a LEFT JOIN different than an ordinary inner JOIN? INNER JOIN takes *all* records from both tables, A and B. Generally, the resultset will be larger. > I ask because I > suspect that your answer will reveal misconceptions about LEFT JOINs > which, when rectified, will cause most of your performance issues to > go away. Maybe my I think too much in "left joining" but I did not know that there is so much difference in performance. Best Regards, Luke _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
