Re: [GENERAL] Understanding Aliases
SELECT-list output aliases name the *output* columns of the SELECT. Those output columns are not available to the SELECT's computation clauses Then it's unclear how could the first query work
Re: [GENERAL] Understanding Aliases
I messed up a little while reformatting, the error message is of course: ERROR: column got_t2_id does not exist _ Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im Auftrag von Stanislav Raskin Gesendet: Dienstag, 11. Dezember 2007 12:33 An: pgsql-general@postgresql.org Betreff: [GENERAL] Understanding Aliases Hello everybody, I did encounter a behaviour of aliases on my postgresql 8.1, which I don't quite understand. Consider this schema layout: CREATE TABLE t1 ( id serial NOT NULL, t2_id integer NOT NULL, t3_id integer NOT NULL, t1_name character varying(255), CONSTRAINT t1_pkey PRIMARY KEY (id), CONSTRAINT t2_id_fkey FOREIGN KEY (t2_id) REFERENCES t2 (id) MATCH SIMPLE ON UPDATE NO ACTION ON DELETE CASCADE, CONSTRAINT t3_id_fkey FOREIGN KEY (t3_id) REFERENCES t3 (id) MATCH SIMPLE ON UPDATE NO ACTION ON DELETE CASCADE ); CREATE TABLE t2 ( id serial NOT NULL, t2_name character varying(255), active boolean NOT NULL, CONSTRAINT t2_pkey PRIMARY KEY (id) ); CREATE TABLE t3 ( id serial NOT NULL, t3_name character varying(255), CONSTRAINT t3_pkey PRIMARY KEY (id) ); CREATE TABLE t4 ( id bigserial NOT NULL, t2_id integer NOT NULL, value integer NOT NULL, CONSTRAINT t4_pkey PRIMARY KEY (id), CONSTRAINT t4_t2_id_fkey FOREIGN KEY (t2_id) REFERENCES t2 (id) MATCH SIMPLE ON UPDATE NO ACTION ON DELETE CASCADE ); Now, I try two different queries. One must select all ids of rows in t2, who have t3 or t1 entries with names containing an 'a', and exactly three entries in t4 with a value of 10. This works quite fine: SELECT t2.id AS got_t2_id FROM t1 JOIN t2 ON (t1.t2_id = t2.id) JOIN t3 ON (t3.id = t1.t3_id) WHERE t2.active AND ( (SELECT COUNT(id) FROM t4 WHERE t2_id = got_t2_id AND value=10) = 3 ) AND ( t3.t3_name ILIKE '%a%' OR t1.t1_name ILIKE '%a%' ) ORDER BY t1.t1_name ASC The second query does not need the names containing 'a' - condition. It only needs to fetch those rows of t2, which have exactly three entries in t4 with a value of 10. Furthermore, no sorting is needed. But, when I try this query SELECT t2.id AS got_t2_id FROM t2 WHERE t2.active AND ( (SELECT COUNT(id) FROM t4 WHERE t2_id = got_t2_id AND value=10) = 3 ) I always get an ERROR: column account_id does not exist What exactly goes wrong here? I simply do not understand, why the first, more complex query works with the alias, but the second one does not. Did I misunderstand the meaning and usage of such aliases as got_t2_id? Thank you very much in advance. Stanislav Raskin
Re: [GENERAL] Understanding Aliases
Note to self: works if I directly reference outer query column name from the inner query. Still no Idea why it does not work with the alias.
Re: [GENERAL] Understanding Aliases
Stanislav Raskin [EMAIL PROTECTED] writes: Still no Idea why it does not work with the alias. SELECT-list output aliases name the *output* columns of the SELECT. Those output columns are not available to the SELECT's computation clauses. Otherwise you'd have a logical circularity --- by definition, the output row(s) can't have been computed yet when you're evaluating a WHERE condition. People frequently think that because ORDER BY can refer to the output-column aliases, other clauses should be able to do it too, but this is not so. Conceptually, ORDER BY happens after computation of the output rows, so there's no logical inconsistency in allowing it to do that ... but it's certainly confused enough people that one wonders whether the SQL committee shouldn't have left that wart out of the language. regards, tom lane ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [GENERAL] Understanding Aliases
People frequently think that because ORDER BY can refer to the output-column aliases, other clauses should be able to do it too, but this is not so. Thank you very much for clearing that one up. Now I have a follow-up question. Let's assume that I want not only to evaluate a subquery for the WHERE-statement, but also want to fetch the result of this subquery. I think that this would be the correct way to do so: SELECT t2.id, (SELECT COUNT(id) FROM t4 WHERE t2_id = t2.id AND value=10) AS t4_num FROM t2 WHERE t2.active AND ( (SELECT COUNT(id) FROM t4 WHERE t2_id = t2.id AND value=10) = 3 ) Will the subquery be evaluated twice, or is postgres smart enough to somehow cache the result from the first call of the subquery? Kind Regards Stanislav Raskin ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [GENERAL] Understanding Aliases
Stanislav, SELECT t2.id, (SELECT COUNT(id) FROM t4 WHERE t2_id = t2.id AND value=10) AS t4_num FROM t2 WHERE t2.active AND ( (SELECT COUNT(id) FROM t4 WHERE t2_id = t2.id AND value=10) = 3 ) select t2.id, count(t4.id) from t2 join t4 using on (t2.id=t4.t2_id) where t2.active and t4.value 10 group by t2.id having count(t4.id) = 3 should do the trick without double select, or? Harald -- GHUM Harald Massa persuadere et programmare Harald Armin Massa Spielberger Straße 49 70435 Stuttgart 0173/9409607 fx 01212-5-13695179 - EuroPython 2008 will take place in Vilnius, Lithuania - Stay tuned!
Re: [GENERAL] Understanding Aliases
should do the trick without double select, or? Indeed it seems to do so. The cost estimation is even about 10 times less with the real application's queries. I never really used group/having even though being aware of their existence. I probably should do so more often. ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [GENERAL] Understanding Aliases
You are quite right, and I must admit that I messed up one more time on this one. I chose an alias, which was already a column name in one of the relations, so the results were rubbish anyhow. Took me some time to figure that out. _ Von: Vyacheslav Kalinin [mailto:[EMAIL PROTECTED] Gesendet: Dienstag, 11. Dezember 2007 19:54 An: Stanislav Raskin Cc: pgsql-general@postgresql.org Betreff: Re: [GENERAL] Understanding Aliases SELECT-list output aliases name the *output* columns of the SELECT. Those output columns are not available to the SELECT's computation clauses Then it's unclear how could the first query work
