sql - relation is unknown inside a subquery - mysql

Question

I have a relation that is built from 2 integers photo_id , user_id and a string -info, (this is the tag) ,
primary key is (user_id, photo_id, info)
photo_id | user_id | info
---------------------------
5 | 3 | aa
7 | 6 | aa
2 | 2 | bb
1 | 2 | cc
1 | 9 | aa
2 | 8 | cc
1 | 4 | cc
9 | 9 | cc
I'm trying to find the k most common tags in my relation.
(secondary sort is by tags).
in this example i would like to get:
k=2 : aa , cc
k=1 : cc
By using this sql query :
SELECT info,tagCount
FROM (SELECT info, COUNT(photo_id) as tagCount
FROM Tags
GROUP BY info
ORDER BY tagCount DESC, info ASC) T
WHERE (SELECT count(info) FROM T T1
WHERE ((T1.tagCount > T.tagCount) OR
(T1.tagCount = T.tagCount AND T1.info < T.info))) < 'k';
But I get the error:
SQL error:
ERROR: relation "t" does not exist
Where is my mistake?

Answer 1

While I still remain unclear on what you are trying to achieve, and assuming the query is for MySQL (not "sql server") then the following may also help. Please note that the cause of the error message is that alias T refers to a resultset, but you cannot reuse that entire resultset in the where clause (the subquery T1 assume that you can reuse T). Regrettablly MySQL (at the time of writing) does not support common table expressions which would allow referencing T like this:
/* T as a common table expression (CTE) */
with T as (
SELECT info, COUNT(photo_id) as tagCount
FROM Tags
GROUP BY info
)
SELECT info,tagCount
, (SELECT count(info) FROM T T1
WHERE (T1.tagCount > T.tagCount) OR
(T1.tagCount = T.tagCount AND T1.info < T.info)
) as k
FROM T
ORDER BY tagCount DESC, info ASC
;
So, in the absence of a CTE capability, you have to repeat the initial subquery, like this:
SELECT
info
, tagCount
, (
SELECT
COUNT(info)
FROM (
SELECT
info
, COUNT(photo_id) AS tagCount
FROM Tags
GROUP BY
info
) T1
WHERE (T1.tagCount > T.tagCount)
OR (T1.tagCount = T.tagCount
AND T1.info < T.info)
)
AS k
FROM (
SELECT
info
, COUNT(photo_id) AS tagCount
FROM Tags
GROUP BY
info
) T
ORDER BY
tagCount DESC
, info ASC
;
and the result of that query (from the sample data) is as follows:
| info | tagCount | k |
|------|----------|---|
| cc | 4 | 0 |
| aa | 3 | 1 |
| bb | 1 | 2 |
Now, exactly how you derive the "expected result" shown in the question (where tag "bb" is not included) I remain unclear.
By the way. Another issue in your original query is that the where clause predicate is comparing an integer to 'k'
where (select count(info) ....) < 'k'
count(info) is an integer, 'k' is a string, so it will fail.

Answer 2

This may only be a step toward your solution as I don't completely understand the question. I think you need to count(distinct column) then use a much simpler where clause.
SQL Fiddle
MySQL 5.6 Schema Setup:
CREATE TABLE Tags
(`photo_id` int, `user_id` int, `info` varchar(2))
;
INSERT INTO Tags
(`photo_id`, `user_id`, `info`)
VALUES
(5, 3, 'aa'),
(7, 6, 'aa'),
(2, 2, 'bb'),
(1, 2, 'cc'),
(1, 9, 'aa'),
(2, 8, 'cc'),
(1, 4, 'cc'),
(9, 9, 'cc')
;
Query 1:
SELECT
info
, COUNT(distinct photo_id) AS photoCount
, COUNT(distinct user_id) AS userCount
FROM Tags
GROUP BY
info
ORDER BY
photoCount DESC
, userCount DESC
, info ASC
Results:
| info | photoCount | userCount |
|------|------------|-----------|
| cc | 3 | 4 |
| aa | 3 | 3 |
| bb | 1 | 1 |