postgres column alias problem
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22-07-2019 - |
Question
As a newbie to Postgresql (I'm moving over because I'm moving my site to heroku who only support it, I'm having to refactor some of my queries and code. Here's a problem that I can't quite understand the problem with:
PGError: ERROR: column "l_user_id" does not exist
LINE 1: ...t_id where l.user_id = 8 order by l2.geopoint_id, l_user_id ...
^
...query:
select distinct
l2.*,
l.user_id as l_user_id,
l.geopoint_id as l_geopoint_id
from locations l
left join locations l2 on l.geopoint_id = l2.geopoint_id
where l.user_id = 8
order by l2.geopoint_id, l_user_id = l2.user_id desc
clause "l.user_id as l_user_id, l.geopoint_id as l_geopoint_id" was added because apparently postgres doesn't like order clauses with fields not selected. But the error I now get makes it look like I'm also not getting aliasing. Anybody with postgres experience see the problem?
I'm likely to have a bunch of these problems -- the queries worked fine in mySql...
Solution
In PostgreSQL you can not use expression with an alias in order by. Only plain aliases work there. Your query should look like this:
select distinct
l2.*,
l.user_id as l_user_id,
l.geopoint_id as l_geopoint_id
from locations l
left join locations l2 on l.geopoint_id = l2.geopoint_id
where l.user_id = 8
order by l2.geopoint_id, l.user_id = l2.user_id desc;
I assume you mean that l2.user_id=l.user_id
ought to go first.
This is relevant message on PostgreSQL-general mailing list. The following is in the documentation of ORDER BY
clause:
Each expression can be the name or ordinal number of an output column (SELECT list item), or it can be an arbitrary expression formed from input-column values.
So no aliases when expression used.
OTHER TIPS
You have:
order by l2.geopoint_id, l_user_id = l2.user_id desc
in your query. That's illegal syntax. Remove the = l2.user_id
part (move it to where
if that's one of the join conditions) and it should work.
Update Below select (with = l2.user_id
removed) should work just fine. I've tested it (with different table / column names, obviously) on Postgres 8.3
select distinct
l2.*,
l.user_id as l_user_id,
l.geopoint_id as l_geopoint_id
from locations l
left join locations l2 on l.geopoint_id = l2.geopoint_id
where l.user_id = 8
order by l2.geopoint_id, l_user_id desc
I ran into this same problem using functions from fuzzystrmatch - particularly the levenshtein function. I needed to both sort by the string distance, and filter results by the string distance. I was originally trying:
SELECT thing.*,
levenshtein(thing.name, '%s') AS dist
FROM thing
WHERE dist < character_length(thing.name)/2
ORDER BY dist
But, of course, I got the error "column"dist" does not exist" from the WHERE clause. I tried this and it worked:
SELECT thing.*,
(levenshtein(thing.name, '%s')) AS dist
FROM thing
ORDER BY dist
But I needed to have that qualification in the WHERE clause. Someone else in this question said that the WHERE clause is evaluated before ORDER BY, thus the column was non-existent when it evaluated the WHERE clause. Going by that advice, I figured out that a nested SELECT statement does the trick:
SELECT * FROM
(SELECT thing.*,
(levenshtein(thing.name, '%s')) AS dist
FROM thing
ORDER BY dist
) items
WHERE dist < (character_length(items.name)/2)
Note that the "items" table alias is required and the dist column alias is accessible in the outer SELECT because it's unique in the statement. It's a little bit funky and I'm surprised that it has to be this way in PG - but it doesn't seem to take a performance hit so I'm satisfied.
"was added because apparently postgres doesn't like order clauses with fields not selected"
"As far as order by goes - yes, PostgresQL (and many other databases) does not allow ordering by columns that are not listed in select clause."
Just plain untrue.
=> SELECT id FROM t1 ORDER BY owner LIMIT 5;
id
30 10 20 50 40 (5 rows)