Re: [GENERAL] Why does this SQL work?
Thank you very much - looks like I will have to prefix all cols. Regards AK On Tue, May 12, 2015 at 3:05 AM, Victor Yegorov wrote: > 2015-05-11 19:26 GMT+03:00 Anil Menon : > >> manualscan=> select count(*) From public.msgtxt where msgid in (select >> msgid From ver736.courier where org_id=3); >> count >> --- >> 10225 >> (1 row) >> >> Please note, there is no msgid col in courier table. Which brings the >> question why does this SQL work? An "select msgid From courier where >> org_id=3" by itself gives error column "msgid" does not exist. >> > > Because you can reference both, inner and outer columns from the inner > query. > Here you're most likely referring to the outer `msgid` in the subquery. > > That's why it is always a good idea to prefix all your columns with tables > aliases. > > > -- > Victor Y. Yegorov >
Re: [GENERAL] Why does this SQL work?
On Tue, May 12, 2015 at 04:07:52PM +0800, Anil Menon wrote: > Thank you very much - looks like I will have to prefix all cols. You should anyway. Queries with unaliased columns make it impossible to analyze without in-depth knowledge of the database. Consider: select c1, c2, c3, c4, c5 from t1 join t2 using (c6) where c7 = 'a' and c8 < now() and c9; which fields belong to which tables? what indexes make sense? it's impossible to tell. if the column references were prefixed with table name/alias - it would become possible, and easy, even, to figure out what's going on. depesz -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Why does this SQL work?
2015-05-11 19:26 GMT+03:00 Anil Menon : > manualscan=> select count(*) From public.msgtxt where msgid in (select > msgid From ver736.courier where org_id=3); > count > --- > 10225 > (1 row) > > Please note, there is no msgid col in courier table. Which brings the > question why does this SQL work? An "select msgid From courier where > org_id=3" by itself gives error column "msgid" does not exist. > Because you can reference both, inner and outer columns from the inner query. Here you're most likely referring to the outer `msgid` in the subquery. That's why it is always a good idea to prefix all your columns with tables aliases. -- Victor Y. Yegorov
Re: [GENERAL] Why does this SQL work?
On Tue, May 12, 2015 at 12:26:15AM +0800, Anil Menon wrote: > manualscan=> select count(*) From msgtxt where msgid in ( > manualscan(> select msgid From courier where org_id=3 > manualscan(> ) > manualscan-> ; > count > --- > 10225 > (1 row) > manualscan=> select count(*) From public.msgtxt where msgid in (select > msgid From ver736.courier where org_id=3); > count > --- > 10225 > (1 row) > Please note, there is no msgid col in courier table. Which brings the > question why does this SQL work? An "select msgid From courier where > org_id=3" by itself gives error column "msgid" does not exist. This works because this is correlated subquery. You should have always use aliases to avoid such errors. Like here: select count(*) From msgtxt as m where m.msgid in ( select c.msgid from courier c where c.org_id = 3 ); Your query is equivalent to: select count(*) From msgtxt as m where m.msgid in ( select m.msgid from courier c where c.org_id = 3 ); which returns all rows from msgtxt if there is at least one row in courier with org_id = 3. depesz -- The best thing about modern society is how easy it is to avoid contact with it. http://depesz.com/ -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
