|
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 |
