Clemens, not any teacher could have explained this any better! Thanks
gert 2012/11/1 Clemens Ladisch <clem...@ladisch.de> > Gert Van Assche wrote: > > 2012/10/31 Clemens Ladisch <clem...@ladisch.de> > >> UPDATE World > >> SET TreeNode = CAST((SELECT COUNT(DISTINCT Continent) > >> FROM World w2 > >> WHERE w2.Continent <= World.Continent) AS TEXT); > > > > this "w2" table you are using, what is this and where do you get it from? > > It's an alias for the World table. (For some reason, I tend to omit > the AS from table aliases; think "FROM World AS w2".) > > "World.Continent" refers to the record in the World table that is > currently being updated. > > The query over w2 is a correlated subquery; it counts how many > continents there are the table before the current record. To understand > it, assume that the UPDATE is currently handling an 'Asia' record, and > consider the results of the following queries: > > SELECT Continent FROM World AS w2 WHERE w2.Continent <= 'Asia'; > SELECT DISTINCT Continent FROM World AS w2 WHERE w2.Continent <= 'Asia'; > SELECT COUNT(DISTINCT Continent) FROM World AS w2 WHERE w2.Continent <= > 'Asia'; > > > Regards, > Clemens > _______________________________________________ > sqlite-users mailing list > sqlite-users@sqlite.org > http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users > _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
